Image forming method and ink composition

ABSTRACT

The present invention provides an image forming method and an ink composition used in the method by which deterioration of a head plate which is formed of silicon is suppressed and more precise images are stably formed, where ink containing an inorganic silicate compound is ejected to form an image from an inkjet head having a nozzle plate where SiO 2  film is provided on a surface thereof at a side toward the ink ejection direction of the nozzle.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 USC 119 from Japanese PatentApplication No. 2009-218010 filed on Sep. 18, 2009, the disclosure ofwhich is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an image forming method in which animage is formed by discharging an ink composition, and an inkcomposition used for the same.

2. Description of the Related Art

In recent years, as a result of ever-increasing demand for theprotection of resources, protection of the environment, enhancement ofoperational stability and the like, the conversion of inks into aqueousproducts has continued to proceed. The product qualities demanded fromaqueous inks include fluidity, storage stability, glossiness of film,clarity, coloring ability and the like, as in the case of oil-based inks

Since most pigments have significantly deteriorated aptitude such aspigment dispersibility with respect to an aqueous vehicle in comparisonwith the case of an oil-based vehicle, sufficient quality cannot beobtained by conventional dispersion methods. The use of variousadditives such as, for example, an aqueous pigment dispersion resin or asurfactant has been studied heretofore. However, an aqueous inkcomparable to an oil-based ink which has existing high quality andsufficient aptitude such as pigment dispersibility has not beenobtained.

With respect to these circumstances, for example, there is disclosed anaqueous ink composition which contains polymer particles and a coloranthaving a water-insoluble polymer coated thereon as a color material (forexample, see Japanese Patent Application Laid-Open (JP-A) No.2001-329199). Further, an aqueous inkjet recording liquid containing apigment and colloidal silica, an ink composition containing a resinemulsion and an inorganic oxide colloid, and the like are disclosed (forexample, see JP-A No. 9-227812, JP-A No. 9-286941, and Japanese PatentNo. 3550637), and a good image can be formed by including colloidalsilica or the like from the viewpoint of abrasion resistance, colorunevenness, clarity, and the like.

Further, there is disclosed an aqueous ink composition which preventsthe elution of glass, silicon, silicon oxide, or the like in contactwith an ink, by using the zeta potential of the ink and the zetapotential between a member and a color material (for example, see JP-ANo. 2003-165936).

SUMMARY OF THE INVENTION

According to an aspect of the invention, an image forming method and anink composition used in the method by which deterioration of a headplate which is formed of silicon is suppressed and more precise imagesare stably formed, where ink containing an inorganic silicate compoundis ejected to form an image from an inkjet head having a nozzle platewhere SiO₂ film is provided on a surface thereof at a side toward theink ejection direction of the nozzle, is provided.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic cross-sectional drawing showing one example ofinner structure of inkjet head.

FIG. 2 is a schematic drawing showing one example of the ejection portarrangement of the nozzle plate.

FIG. 3 is a plane perspective diagram showing one example of headstructure.

FIG. 4 is a magnified drawing magnifying and showing a part of FIG. 3.

FIG. 5 is a magnified drawing magnifying and showing a part of twodimensional nozzle arrangement (32×64).

FIG. 6 is a perspective diagram magnifying and showing a part of anotherexample of the head structure.

FIG. 7 is a plane perspective diagram magnifying and showing a part ofthe pressure chamber.

FIG. 8 is a cross-sectional drawing along line 9-9 of FIG. 7.

DETAILED DESCRIPTION

Among the above-described prior arts, abrasion resistance of the formedimage in an aqueous ink composition including a polymer particle isimproved, however, it was difficult to say that the composition issufficient from the viewpoint of ejection properties.

Further, it is known that in the case where an inkjet head contacts ink,deterioration of the head member is liable to be caused by the effectsof the ink. In particular, the tendency tends to be largely shown whenthe aqueous ink contains polymer particles.

The invention has been made in view of the above circumstances, and theobject of the invention is to provide an image forming method and an inkcomposition where deterioration of the head plate is suppressed tostably form higher precise images. It is a goal of this invention toachieve this object.

According to the invention, when the nozzle plate of an inkjet head isformed of silicon, the silicon portion, for example a portion ofsilicon, silicon oxide or the like, is eluted by contact with ink andeasily eroded. With respect to this situation, when an ink compositioncontains an inorganic silicate compound, there is an effect whereerosion of the silicon portion is prevented. In particular, this effectis prominent when using a structure where plural nozzles (ejection port)are arranged two-dimensionally for the movement of a large volume of inkto the nozzle, such that higher precise images of, for example 1200 dpi,or the like are obtained. The present invention is accomplished on thebasis of this knowledge.

Exemplary embodiments according to the aspect of the invention include,but are not limited to the following items <1> to <18>.

-   <1> An image forming method including: discharging an ink    composition from an inkjet head to form an image, the ink    composition comprising an inorganic silicate compound, and the    inkjet head being equipped with a nozzle plate at least a part of    which is formed of silicon.-   <2> The image forming method according to the item<1>, wherein at    least a part of the nozzle plate is coated with a film which    includes at least one selected from the group consisting of a metal    oxide, a metal nitride and a metal other than silicon.-   <3> The image forming method according to the item<1> or the    item<2>, wherein at least a part of the nozzle plate is coated with    a film which comprises SiO₂ or tantalum oxide.

<4> The image forming method according to any one of the item<1> to theitem <3>, wherein the nozzle plate has plural ejection ports which ejectthe ink composition, the inkjet head further includes plural pressurechambers respectively communicating with the plural ejection ports ofthe nozzle plate, plural ink supply flow paths respectively supplyingthe ink composition to the plural pressure chambers, a common liquidchamber supplying the ink composition to the plural ink supply flowpaths, and plural pressure generation units respectively deforming theplural pressure chambers, and an amount of change in volume within eachpressure chamber is controlled by driving the respective pressuregeneration unit to eject the ink composition.

-   <5> The image forming method according to the item<4>, wherein the    pressure generation units are piezo elements.-   <6> The image forming method according to the item<4> or the    item<5>, wherein the plural ejection ports are arranged    two-dimensionally in a matrix form.-   <7> The image forming method according to the item<6>, wherein the    inkjet head forms an image at a drawing resolution of 1200 dpi or    higher with a single pulse-type ejection from the nozzle plate.-   <8> The image forming method according to any one of the item<4> to    the item<7>, further including electrical wiring which is arranged    so as to penetrate the common liquid chamber and supplies driving    signals to the pressure generation units.-   <9> The image forming method according to the item<8>, wherein the    pressure generation units are disposed on the opposite side of the    pressure chamber from a side thereof where the nozzle plate is    arranged and the common liquid chamber is disposed on the opposite    side of the pressure generation units from a side thereof where the    pressure chamber are arranged.

<10> The image forming method according to any one of the item<1> to theitem <9>, wherein a content of the inorganic silicate compound in theink composition is 0.0005% by mass to 0.5% by mass with respect to atotal mass of the ink composition.

-   <11> The image forming method according to any one of the item<1> to    the item<10>, wherein the ink composition further includes a    pigment, a water-soluble organic solvent, and resin particles.-   <12> The image forming method according to any one of the item<1> to    the item<11>, wherein a pH of the ink composition is in a range of    from 7.5 to 10.0 at a temperature of 25° C.-   <13> The image forming method according to any one of the item<1> to    the item<12>, wherein the resin particles are self-dispersing    polymer particles.-   <14> The image forming method according to any one of the item<1> to    the item<13>, wherein the inorganic silicate compound is colloidal    silica.

<15> The image forming method according to any one of the item<1> to theitem <14>, further including applying a treatment liquid on a recordingmedium, the treatment liquid being capable of forming an aggregate uponcontact with the ink composition.

-   <16> The image forming method according to the item<15>further    including drying the treatment liquid on the recording medium by    heating, during a period from the treatment liquid is applied onto    the recording medium until the ink composition is applied.-   <17> The image forming method according to the item<16>, wherein a    heating temperature for drying the treatment liquid on the recording    medium by heating is in a range from 40° C. to 150° C.-   <18> An ink composition including an inorganic silicate compound and    being used for the image forming method according to any one of the    item<1> to the item<17>.

An image forming method of the invention is described in detail below,and an ink composition is described below through the description.

The image forming method of the invention is constituted to include aprocess (hereinafter, “ink ejection process”) where the ink compositionincluding an inorganic silicate compound is ejected from an inkjet headhaving a nozzle plate including silicon in at least one part to form animage, and can be constituted to provide another process, if necessary.The invention further preferably has a process (hereinafter, “treatmentliquid providing process”) which provides a treatment liquid on arecording medium to be able to form an aggregate upon contact with theink composition.

In the invention, the ink composition in contact with the nozzle plateformed of silicon of the inkjet head is constituted to contain aninorganic silicate compound, and therebythe portion formed of silicon ofthe nozzle plate can be prevented from deteriorating due to erosion.Therefore, the size of ink droplets or the ejection rate duringejection, or the like is changed to suppress ejection defect from beingcaused, and high quality image can be stably formed.

[Ink Ejection Process]

The ink ejection process uses an inkjet head which has a nozzle plateincluding silicon in at least one part and an ink composition(hereinafter, simply referred to “ink”) including an inorganic silicatecompound is ejected to form an image. In this process, the inkcomposition can be provided selectively on the recording medium and canform a desirable visible image.

Image recording utilizing the inkjet method can be performed,specifically, by supplying energy thereby ejecting a liquid compositionto a desired recording medium, that is, plain paper, resin-coated paper,paper used exclusively for inkjet recording described, for example, inJP-A Nos. 8-169172, 8-27693, 2-276670, 7-276789, 9-323475, 62-238783,10-153989, 10-217473, 10-235995, 10-337947, and 10-217597, films, commonuse paper for electrophotography, clothes, glass, metals, ceramics, etc.As the inkjet recording method suitable to the invention, a methoddescribed in JP-A No. 2003-306623, in columns (0093) to (0105) may beapplicable.

The inkjet method is not particularly limited and may be of any knownsystem, for example, a charge control system of ejecting an inkutilizing an electrostatic attraction force, a drop on demand system ofutilizing a vibration pressure of a piezo element (pressure pulsesystem), an acoustic inkjet system of converting electric signals intoacoustic beams, irradiating them to an ink, and ejecting the inkutilizing a radiation pressure, and a thermal inkjet system of heatingan ink to form bubbles and utilizing the resultant pressure (BUBBLEJET(registered trade mark)). Examples of the inkjet method include a systemof injecting a number of ink droplets of low concentration, a so-called“photo-ink” each in a small volume, a system of improving an imagequality using plural inks of a substantially identical hue and ofdifferent concentrations, and a system of using a colorless transparentink.

In the invention, there is preferred a method where a pressuregeneration unit (for example, piezo element) using the pressure pulsemethod is used, the pressure generation unit is driven to control anamount of change in volume within each pressure chamber and thereby thedroplet diameter of the ink composition to be ejected from the nozzle ischanged to eject the ink composition from the nozzle; and a method wherethe pressure generation unit is driven many times, to thereby controlthe number of droplets ejected from the nozzle, and plural droplets arecombined before landing. In this case, it is more important to suppresserosion of the silicon portion of the nozzle plate due to ink. Amultiple tone image can be stably recorded with the ink composition ofthe invention.

The inkjet head used in the inkjet method may be either an on-demandsystem or a continuous system. The ejection system includes,specifically, for example, an electric-mechanical conversion system (forexample, single cavity type, double cavity type, bender type, pistontype, share mode type, and shared wall type, etc.), an electric-thermalconversion system (for example, thermal inkjet type, BUBBLEJET(registered trade mark) type, etc.), an electrostatic attraction system(for example, electric field control type, and slit jet type, etc.), andan electric ejecting system (for example, spark jet type, etc.) and anyof the ejection systems may be used.

Ink nozzles, etc. used for recording by the inkjet method are notparticularly limited but can be selected properly depending on thepurpose.

As an inkjet head, there are a shuttle type where a short serial head isused to record while scanning the head in the width direction of therecording medium and a line head type (single-pass type) where arecording device is arranged in correspondence with the entire area ofone side of the recording medium and the line head is used. Thesingle-pass type forms an image on the whole face of a recording mediumdue to an operation where a full line head and the recording medium arerelatively moved once, using the full line head which covers the wholearea of the recording medium. For example, the single-pass type isdescribed in JP-A Nos. 2005-96443, and 2005-280346. That is to say, thesingle-pass type can perform image recording on the whole face of therecording medium by scanning the recording medium in the directionorthogonal to the device arrangement direction of the full line head,and a transferring system such as a carriage which scans the short headis not necessary. Further, since a complicated scanning control of themovement of the carriage and the recording medium is not necessary andonly the recording medium is moved, a high recording rate can berealized in comparison with the shuttle type. The image forming methodof the invention can be applied to all these types; however, when themethod is generally applied to the single-pass type, a high precisenozzle arrangement and high ejection frequency are required, andtherefore suppression of erosion due to ink is more important. There isa large effect on the improvement of ejection precision due to inkcomposition of the invention and prevention of erosion due to contact ofthe ink with the nozzle plate.

Furthermore, in the ink discharging (ejecting) step according to theinvention, when a line method is employed, recording can be suitablyperformed not only using one type of the ink composition, but also usingtwo or more types of ink compositions, by setting the ejection (dropletejection) interval between the first ejected ink composition (n-th color(n≧1), for example, the second color) and the subsequently ejected inkcomposition ((n+1)-th color, for example, the third color), at 1 secondor less. According to the invention, by setting the ejection interval at1 second or less in the line method, images having excellent abrasionresistance and suppressed occurrence of blocking can be obtained underhigh speed recording that is faster than that conventionally obtained,while preventing the spreading caused by the interference between inkdroplets or mixed colors. Further, images having excellent hue anddrawing properties (reproducibility of fine lines or fine parts in theimage) can be obtained.

The volume of a droplet to be ejected from the inkjet head is preferably0.5 to 12 pL (picoliter) from the viewpoint of obtaining a high preciseimage. Further, a method where plural ink droplet volumes are combinedto form an image is preferred with respect to correction of unevennessor stripes during formation of a highlight image. In this case, thevolume of the small droplets forming the highlight image is preferably0.5 to 4 pL, the volume of the medium droplets to be mainly used ispreferably 2 to 8 pL, and the volume of the large droplets to be used inthe correction of unevenness or stripe is preferably 6 to 12 pL.

(Inkjet Head Having Silicone Nozzle Plate)

The inkjet head employed in the image forming method has a nozzle plate.At least a part of the nozzle plate contains silicone. FIG. 1 is aschematic diagram showing one example of an internal structure of theinkjet head.

FIG. 1 shows an inkjet head 200 having a nozzle plate 11 and an inksupplying unit 20 which is provided on a opposite side from the inkejecting direction of the nozzle plate. The nozzle plate 11 has pluralejection openings 12 through which inks are ejected.

As typically shown in FIG. 2, the ejector ports (nozzles) (32×64) arearranged two-dimensionally in the nozzle plate 11. The nozzle plate isformed in part or on entirely of silicon. A structure may be used wheresilicon is exposed within a nozzle port and on the surface thereof at aside toward the ink ejection direction of the nozzle, which arepreferably coated with a film which contains at least one kind selectedfrom the group consisting of metal (including silicon) oxide andnitride, and a metal (other than silicon).

Further, the surface of the nozzle plate at a side toward the inkejection direction of the nozzle may be coated with a liquid repellentfilm in order that wettability with respect to ink is suppressed toprevent ink pollution in the vicinity of nozzle. As the liquid repellentfilm, a film including fluorocarbon is preferably used.

High quality image recording can be performed with a high resolution of1200 dpi by high speed single-pass (one pass of the recording medium)due to the nozzle plate. That is to say, plural nozzles of the nozzleplate are disposed two-dimensionally in a matrix form, and an ink supplyunit which is fixed to the nozzle has the flow path configurationallowing large volumes of ink to be ejected with high frequency (ejectedwith so-called high duty). Silicon, which is easily used in asemiconductor process, is used in part or in the whole in order toobtain a high precise image. Specifically, when a part or the whole ofthe nozzle plate is formed of silicon, for example, single crystalsilicon and polysilicon can be used as silicon. In the nozzle plateformed of silicon, erosion due to ink is recognized as a generalproblem, and erosion prevention using various protective films can beexamined. However, it is very difficult task to completely preventerosion due to ink of the protective film itself or erosion due to inkresulting from defects, or the like in the protective film, such assilicon oxide. In particular, as the frequency of ink ejection, such asin the high speed single-pass type, is high and fresh ink readilycontacts the silicon and the protective film at all times, erosion ofthe silicon and the protective film due to ink easily proceeds. Further,in the high speed single-pass type where high precision is required,there is high level of a demand for a response to the deterioration ofejection precision due to ink erosion.

In the invention, the ink composition to be used in ejection contains aninorganic silicate compound, and thereby the deterioration of the easilyeroded silicon can be effectively prevented.

The nozzle plates can be coated by forming a film which contains atleast one kind selected from the group consisting of metal (includingsilicon) oxide and nitride, and metal (excluding silicon). Specifically,when a part or the whole of the nozzle plate is formed of silicon, forexample, single crystal silicon and polysilicon. Further, when a part orthe whole of the nozzle plate is formed of silicon, for example, theremay be provided a film such as a metal oxide, for example silicon oxide,titanium oxide, chromium oxide, or the like or metal nitride such astitanium nitride, silicon nitride, or the like, or metal such aszirconium, on the single crystal silicon substrate. The silicon oxidemay be, for example, SiO₂ film formed by oxidizing the whole or a partof the silicon surface of the nozzle plate formed of silicon. A filmsuch as tantalum oxide (preferably, such as tantalum pentoxide (Ta₂O₅))or zirconium, chromium, titanium, glass, or the like may be formed on apart or the entirety of the silicon surface. Further, a part of thesilicon may be constituted to be replaced with glass (for example,borosilicate glass, photosensitive glass, quartz glass, soda-limeglass). A film consisting of tantalum pentoxide, or the like as well astantalum oxide has excellent ink resistance; in particular good erosionresistance with respect to alkaline ink is obtained.

An embodiment of the method forming the SiO₂ film is described. Forexample, SiCl₄ and water vapor is introduced into a chemical vapordeposition (CVD) reactor in which an uncoated silicon substrate isprovided, and thereby an SiO₂ film can be formed on the siliconsubstrate. After a valve between a CVD chamber and a vacuum pump pumpsout fluid and empties the chamber, the valve is closed, and SiCl₄ andH₂O vapor are introduced to the chamber. The partial pressure of SiCl₄can be set between 0.05 and 40 torr (6.67 to 5.3×10³ Pa) (for example,0.1 to 5 torr (13.3 to 666.5 Pa), and the partial pressure of H₂O can beset between 0.05 and 20 torr (for example 0.2 to 10 torr). Thedeposition temperature is generally between room temperature and 100° C.Further, in another embodiment, SiO₂ film can be formed on the siliconsubstrate by sputtering. The surface to be coated with SiO₂ film ispreferably cleaned before forming the SiO₂ film (for example, byapplying oxygen plasma).

The configuration example of the inkjet head including the nozzle platehaving plural ejection ports (nozzles) which are arrangedtwo-dimensionally is described in reference to FIG. 3 to FIG. 4. FIG. 3is a plane perspective diagram showing one example of the headstructure, and FIG. 4 is a magnified drawing magnifying and showing apart of FIG. 3.

In order to densify dot pitch recorded on the recording medium, it isnecessary that the nozzle pitch is densified in head 50. The head 50 hasa structure where plural ink chamber units 104 which consist of nozzle100 ejecting the ink droplets and pressure chamber 102 corresponding tothe nozzle 100 is disposed in zigzag in a matrix form, as showed inFIGS. 3 and 4. Thereby, an apparent densified nozzle pitch is attempted.That is to say, the head 50 is a full line head which provides at leastone nozzle row where the plural nozzles 100 ejecting ink are arrangedover the length corresponding to the whole width of the recording mediumin the direction (principal scanning direction) substantially orthogonalto the transfer direction (sub-scanning direction) of the recordingmedium, as shown in FIGS. 3 and 4.

One example of a case where ink is ejected from the nozzle plate havingthe plural nozzles is described in reference to FIG. 5. In FIG. 5, fourrows of nozzle rows from 311 to 341 are shown, but substantially, thetotal of 64 rows are disposed in one head module with a repeatedarrangement pattern in the same manner as the four rows. 32 nozzles arearranged in each nozzle row. In FIG. 5, Y direction is paper transferdirection (sub-scanning direction), and X direction is longitudinaldirection (principal scanning direction) of the line head. When oneprincipal scanning line 260 is ejected, dot 314 is ejected from nozzle312 of nozzle row 311. The dot 324 adjacent to dot 314 in the principalscanning direction is ejected from nozzle 332 of nozzle row 331 in thenext two rows with respect to nozzle row 311. The dot 334 adjacent todot 324 in the principal scanning direction is ejected from nozzle row322 of nozzle row 321 adjacent to nozzle row 311. The dot 344 adjacentto dot 334 in the principal scanning direction is ejected from nozzle342 of nozzle row 341 in the next three rows with respect to nozzle row311. Thus, four nozzle rows are used one by one in the prescribedpattern, and the adjacent dot (for example, a group of adjacent fourdots such as 362 or 364 in FIG. 5) in the principal scanning directionis ejected.

In FIG. 1, the ink supply unit 20 is equipped with plural pressurechambers 21, which respectively communicate with the plural ejectionopenings (ejection ports) 12 of the nozzle plate 11 through the nozzlecommunication path 22, plural ink supplying paths 23 that respectivelysupply ink to the plural pressure chambers 21, a common liquid chamber25 that supplies ink to the plural ink supplying paths 23, and apressure generation unit 30 that respectively transforms the pluralpressure chambers 21.

The ink supplying paths 23 locate between the ink supply unit 20 and thenozzle plate 11, and an ink which has been supplied to the common liquidchamber 25 is introduced to the ink supplying path 23. One terminal of asupply regulating path 24 which is connected with the pressure chambers21 is connected to the ink supplying path 23 so that an amount of an inksupplied from the ink supplying path 23 to the pressure chamber 21 whichis located adjacent to the pressure generation unit 30 may be regulatedto be a desired one. This system may enable to supply a plenty of amountof ink to the plural ejection openings.

The pressure generation unit 30 is an actuator (piezo device) which isconstituted by sequentially stacking a vibration plate 31, an adhesivelayer 32, a lower electrode 33, a piezoelectric body layer 34, and anupper electrode 35, from the pressure chamber 21 side. The pressuregeneration unit 30 is such that an electrical wire supplying a drivingsignal from the exterior is connected to be driven. The piezoelectricbody layer is joined with electrode on the vibration plate (pressingplate) 31 which constitutes the upper face of pressure chamber 21. Theactuator is deformed according to an image signal by applying anelectric voltage to the electrode. The ink is ejected from the nozzlethrough the nozzle communicating path. When the ink is ejected, new inkis supplied to the pressure chamber 21 through the ink supply path 23from the common liquid chamber 25.

A circulation aperture 41 which continuously collects an ink to acirculation path 42 is provided in the vicinity of the ejection opening12. Increase of viscosity of an ink in the vicinity of the ejectionopening during non-driving period may be suppressed thereby.

FIG. 6 is a perspective diagram showing another preferable example ofthe inner structure of inkjet head. FIG. 7 is a plane perspectivediagram magnifying and showing a part of the pressure chamber. FIG. 8 isa cross-sectional drawing along line 9-9 of FIG. 7.

In order to densify the nozzle pitch, the inkjet head has the nozzleplate 194, plural pressure chambers 152 respectively communicating withthe plural nozzles (ejection port) 151 of the nozzle plate, the pluralink supply flow paths 153 which supply ink to the plural pressurechambers 152, a common liquid chamber 155 which supplies the inkcomposition to each of the plural ink supply flow paths 153, pressuregeneration units 158 which respectively deform the plural pressurechambers 152, and electrical wirings 190 which respectively supplydriving signals to the pressure generation units 158.

In order to obtain higher precise images, the configuration of theinkjet head, shown in FIGS. 6 to 8, in part or as a whole uses silicon,which is easily used in a semiconductor process, and the rear face flowpath design is used as flow path configuration where large volumes ofink can be ejected with high frequency. Through the rear face flow pathdesign, large volumes of ink can be supplied to the nozzle disposed suchthat higher precise images can be formed. As a result, fresh ink readilycontacts the nozzle plate at all times. Even though the protective filmis formed on the nozzle plate surface or inside the nozzle plate,silicon erosion in the nozzle plate main body easily proceeds throughcontact with ink due to ink penetration at a defective portion, or thelike of film. In the invention, through the addition of an inorganicsilicate compound to ink composition used in ejection, deterioration ofsilicon, which is likely to be eroded, can be effectively prevented.

In the head 150, vibration plate 156, which forms the upper face ofpressure chamber 152, is disposed on the upper side of the pressurechamber 152 having nozzle 151 and ink supply flow path 153. Thepiezoelectric device 158 (piezo actuator) is disposed as the pressuregeneration unit which consists of a piezoelectric body, such as a piezowhich sandwiches an electrode at the upper and lower sides of portioncorresponding to each pressure chamber 152 on vibration plate 156. Thepiezoelectric body 158 has separate electrode 157 on the upper face. Theelectrode pad 159 is drawn and formed as an electrode connecting sectionfrom the edge face of the separate electrode 157 to outside. Theelectrical wiring 190 on the electrode pad 159 is stood substantiallyvertically to the face including the piezoelectric device 158 (pressuregeneration unit). The multilayer flexible cable 192 is disposed on theelectrical wiring 190 which is stood substantially vertically withrespect to the face including the piezoelectric device 158, and thedriving signal is supplied through the wiring from head driver to theseparate electrode 157 of the piezoelectric device 158.

The space of the columnar electrical wiring (electrical column) 190,between where the vibration plate 156 and the flexible cable 192 arealigned, is a common liquid chamber 155 for supplying ink to pressuregenerating chamber 152 through the ink supply flow path 153 from hereto.

The electrical wiring 190 which is stood as a vertical column on theelectrode pad 159 drawn from the separate electrode 157 to pressurechamber 152 supports the flexible cable 192 from below, and the space ofthe common liquid chamber 155 is formed. The electrical wiring 190 isformed to penetrate the common liquid chamber 155. Further, theelectrical wiring 190 is formed with respect to the piezoelectric device158 (of the separate electrode 157) one by one and is corresponded toone-on-one. However, in order to decrease number of wirings (electricalcolumn number), one electrical wiring 190 may correspond to pluralpiezoelectric devices 158 such that the wirings for some piezoelectricdevices 158 collectively form one electrical wiring 190. Further, wiringfor not only separate electrodes 157 but a common electrode (vibratingplate 156) may be formed as the electrical wiring 190.

As shown in FIG. 6, the nozzle 151 is formed on the bottom face, and theink supply flow path 153 is provided at the upper face of the diagonalcorner to the nozzle 151. The ink supply flow path 153 penetrates thevibrating plate 156 and the common liquid chamber 155 and the pressurechamber 152 thereon is directly connected through the ink supply flowpath 153. Thereby, the common liquid chamber 155 and the pressurechamber 152 can be directly fluidically connected.

The vibrating plate 156 is common to each pressure chamber 152 and formsone plate. The piezoelectric device 158 for deforming the pressurechamber 152 is disposed in the portion corresponding to the pressurechamber 152 of the vibrating plate 156. The electrode (common electrodeand separate electrode) for driving the device by applying the electricvoltage to the piezoelectric device 158 is formed at the upper and lowerfaces so as to sandwich the piezoelectric device 158.

The vibrating plate 156 may be formed of a conductive thin film such as,for example SUS, or the like to serve as a common electrode. In thiscase, in order that the piezoelectric device 158 is individually driven,a separate electrode 157 is formed in the upper face of piezoelectricdevice 158.

As described above, the electrode pad 159 is drawn from the separateelectrode 157, and the electrical wiring 190 (electric column) whichstands vertically on the electrode pad 159, and penetrates the commonliquid chamber 155 is formed. In the electrical wiring 190 (electriccolumn), the electrical wiring 190 is formed in a tapered shape duringthe production process as shown in FIG. 7.

The multilayer flexible cable 192 is formed on columnar electricalwiring 190. The electrical wiring 190 is a column to support themultilayer flexible cable 192, the vibrating plate 156 is used as thefloor, the multilayer flexible cable 192 is used as the ceiling, and thespace for the common liquid chamber 155 is secured. Further, theelectrical wirings 190 are respectively connected to separate wirings(not shown) to supply driving signals to each electrodes 157, andthereby piezoelectric devices 158 are driven.

FIG. 7 shows a plane perspective diagram magnifying a part of thepressure chamber 152. As described above, the pressure chamber 152 issubstantially square in shape, and the nozzle 151 and the ink supplyflow path 153 are formed at diagonally opposed corners to each other,the electrode pads 159 is drawn to the nozzle 151 side, and theelectrical wirings (electrical column) 190 are formed thereon.

As shown in FIG. 8, the head 150 is formed by laminating plural thinfilms, or the like.

The flow path plate 196 is laminated where the nozzle flow path 151 a orthe like, which connects the pressure chamber 152, the ink supply port153 and the pressure chamber 152 with the nozzle 151, is formed on thenozzle plate 194 forming the nozzle 151. Here, the flow path plate 196is represented as one plate although, in practice, the flow path plate196 may be formed by laminating plural plates.

Further, a part or the whole of the nozzle plate 194 is formed ofsilicon. A structure may be used where the silicon is exposed within thenozzle port and on a surface thereof at a side toward the ink ejectiondirection of the nozzle, which are preferably coated with a film whichcontains at least one kind selected from the group consisting of metal(including silicon) oxide, metal nitride, and metal (excluding silicon).

Further, the surface thereof at a side toward the ink ejection directionof the nozzle plate may be coated with a liquid repellent film in orderthat wettability due to ink is suppressed to prevent ink stain in thevicinity of nozzle. As the liquid repellent film, a film includingfluorocarbon is preferably used.

The vibration plate 156 forming the upper face of the pressure chamber152 is laminated on the flow path plate 196. The vibration plate 156preferably serves as a common electrode for driving the piezoelectricdevice 158 together with the separate electrode 157. Further, an openingcorresponding to ink supply flow path 153 of the pressure chamber 152 isprovided in the vibrating plate 156, to thereby directly communicate thecommon liquid chamber 155 formed on the vibrating plate 156 with thepressure chamber 152

The piezoelectric body 158 a is formed in the portion corresponding tosubstantially the entire face of the upper face of the pressure chamber152 on the vibrating plate 156 (common electrode) and the separateelectrode 157 is formed in the upper face of the piezoelectric body 158a.

The piezoelectric body 158 a, which is interposed between the commonelectrode (vibrating plate 156) and separate electrode 157 on the upperand lower side, modifies the pressure chamber 152 to reduce the volumethereof when an electric voltage is applied by the common electrode 156and the separate electrode 157, and constitutes a piezoelectric device158 (piezoelectric actuator) for ejecting ink from the nozzle 151.

The edge of the nozzle 151 of the separate electrode 157 forms theelectrode pad 159 as the electrode connecting section which is drawnoutward. The columnar electrical wiring 190 (electric column) is formedvertically on the electrode pad 159 so as to penetrate the common liquidchamber 155.

The multilayer flexible cable 192 is formed on the electrical wiring190. Each wiring (not shown) formed in the multilayer flexible cable 192connects the electrode pad 190 a to the electrical wiring 190, a drivingsignal is supplied through the electrical wiring 190 in order to driveeach piezoelectric device 158.

The space where the columnar electrical wiring 190 (electric column)between the vibration plate 156 and the multilayer flexible cable 192 isaligned is the common liquid chamber 155 filled with ink in order tosupply ink to the pressure chamber 152 which pools ink. Here, in orderto fill the ink, insulation and protective films 198 are formed on theink contacting surfaces of the vibrating plate 156, the separateelectrode 157, the piezoelectric body 158 a, and the electrical wiring190, as well as the multilayer flexible cable 192.

The common liquid chamber which has been conventionally on the same sideas the pressure chamber with respect to the vibration plate is disposedhere on the vibration plate and is provided in opposition to thepressure chamber. Therefore, the conventionally required pipe, or thelike for introducing ink to the pressure chamber from the common liquidchamber is not necessary. Further, the size of the common liquid chambercan be increased, and thereby ink can be satisfactorily supplied anddensification of the nozzles can be attained. With the achievement ofdensification, the nozzles can be driven at a high frequency. Wiring tothe separate electrode of each piezoelectric device is stood verticallyfrom the electrode pad of the separate electrode, and is made topenetrate the common liquid chamber. Therefore, the wiring for supplyinga driving signal to each piezoelectric device can be densified. Further,the common liquid chamber is disposed on the vibrating plate, the commonliquid chamber and pressure chamber is directly connected through theink supply port. Thereby, the common liquid chamber and pressure chambercan be directly fluidically connected, and the common liquid chamber isdisposed on the vibrating plate. Therefore, the length of the nozzleflow path 151 a from the pressure chamber 152 to nozzle 151 can beshortened over conventional methods. Even in the case of densification,high viscosity ink (for example, about 20 cp to 50 cp) can be ejected.Further, a flow path structure can be made which is able to promptlyrefill after ejection.

Further, the inner structure of inkjet head, shown in FIGS. 6 to 8 isdescribed in [0090] to [0113] of JP-A No. 2006-111000.

[Treatment Liquid Applying Step]

In an image forming method of the invention, a treatment liquid applyingstep may be provided, which performs imaging by applying a treatmentliquid configured to form aggregates when contacted with the inkcomposition, to a recording medium, and placing the treatment liquid incontact with an ink composition. In this case, dispersed particles ofthe polymer particles or coloring material (for example, pigment) in theink composition aggregate, and an image is fixed to the recordingmedium. In addition, the details and preferred embodiments of therespective components in the treatment liquid are as describedpreviously.

Application of the treatment liquid may be performed by applying knownmethods such as a coating method, an inkjet method, and an immersionmethod. The coating method may be performed by a known coating methodusing a bar coater, an extrusion die coater, an air doctor coater, abread coater, a rod coater, a knife coater, a squeeze coater, or areverse roll coater. Details of the inkjet method are as describedabove.

The treatment liquid discharging step may be provided before or afterthe ink applying step using the ink composition.

In the invention, an embodiment in which the ink discharging step isprovided after the treatment liquid is applied in a treatment liquidapplying step, is preferable. That is, an embodiment in which, beforeapplication of the ink composition on the recording medium, a treatmentliquid for aggregating a coloring material (preferably pigment) in theink composition is applied in advance, and the ink composition isapplied so as to contact the treatment liquid applied on the recordingmedium to form an image, is preferable. Thereby, inkjet recording may bespeeded-up and, even when high speed recording is performed, an imagehaving high density, and high resolution is obtained.

The amount of application of the treatment liquid is not particularlylimited so long as the liquid can aggregate the ink composition, but canbe an amount resulting in an amount of application of the aggregatedcomponent (for example, a carboxylic acid or a cationic organic compoundhaving a valency of 2 or greater) of 0.1 g/m² or more. Among them, anamount resulting in an amount of application of the aggregated componentof 0.1 to 1.0 g/m² is preferred, and an amount resulting in 0.2 to 0.8g/m² is more preferred. When the amount of application of the aggregatedcomponent is 0.1 g/m² or more, the aggregation reaction proceedssatisfactorily, and when the amount is 1.0 g/m² or less, the glossinessis not very high, and is preferable.

According to the invention, it is preferable to provide an inkdischarging step after the treatment liquid applying step, and tofurther provide a heating drying step of heating and drying thetreatment liquid on the recording medium, during a period from afterapplying the treatment liquid onto the recording medium until the inkcomposition is applied. By heating and drying the treatment liquidpreviously before the ink discharging step, ink coloring properties suchas the prevention of spreading becomes good, and visible images havinggood color density and hue can be recorded.

The heating and drying can be carried out by a known heating means suchas heater, an air blowing means utilizing air blowing such as dryer, ora means combining these. Examples of the heating method include a methodof supplying heat by a heater or the like, from the surface of therecording medium opposite the surface applied with the treatment liquid,a method of blowing a warm air or hot air to the surface of therecording medium applied with the treatment liquid, a method of heatingusing an infrared heater, or the like. Heating can also be performed bycombining these methods.

[Heating Fixing Step]

It is preferable that the image forming method of the inventionincludes, after the ink applying step, a heating fixing step for heatingand fixing the ink image formed by the application of the inkcomposition by placing the image in contact with a heated surface. Byadding a heating fixing treatment, fixing of the image on the recordingmedium is achieved, and the resistance of the images to abrasion can befurther enhanced.

Heating can be preferably performed at the glass transition temperature(Tg) or higher of the polymer particle in the image. Since heating isperformed at the Tg temperature or higher, the film is formed tostrengthen the image. The heating temperature is preferably in thetemperature region of Tg+10° C. or higher. Specifically, the heatingtemperature is preferably in a range of from 40° C. to 150° C., morepreferably in a range of from 50° C. to 100° C., and even morepreferably in a range of from 60° C. to 90° C.

From the viewpoint of surface smoothing, a pressure duringpressurization together with heating is preferably in a range of from0.1 MPa to 3.0 MPa, more preferably in a range of from 0.1 MPa to 1.0MPa, and even more preferably in a range of 0.1 MPa to 0.5 MPa.

The heating method is not particularly limited, but methods ofnon-contact drying such as a method of heating with a heat generatorsuch as a nichrome wire heater; a method of supplying warm air or hotair; and a method of heating with a halogen lamp, an infrared lamp orthe like, may be suitably exemplified. The method of heating andpressing is not particularly limited, but methods of performing heatingand fixing by contact such as, for example, a method of pressing a heatplate to the image-formed surface of the recording medium, and a methodof passing the image through a pair of rollers using a heating andpressing apparatus equipped with a pair of heating and pressing rollers,a pair of heating and pressing belts, or a heating and pressing beltdisposed on the side of the image-recorded surface of the recordingmedium and a retaining roller disposed on the opposite side, may besuitably mentioned.

In the case of heat and pressing, a NIP time of 1 msec to 10 sec ispreferable, more preferable is 2 ms to 1 s, and even more preferable is4 msec to 100 msec. Further, a NIP width of 0.1 mm to 100 mm ispreferable, more preferable is 0.5 mm to 50 mm, and even more preferableis 1 mm to 10 mm.

As the heating and pressing roller, a metal roller made from metal or aroller having a coating layer including an elastic body around a metalbar core and a surface layer (referred to as separate layer) provided ifnecessary, may be used. For example, the latter bar core can beconstituted by a cylindrical body made from iron, aluminum, SUS, or thelike. The surface of the bar core is preferably coated with the coatinglayer at least in part. In particular, the coating layer may bepreferably formed of a silicon resin or fluorocarbon resin having moldreleasability. Further, a heat generation unit is preferably built intothe bar core of one side of the heating and pressing roller, and heatingand pressing process are simultaneously preformed by passing therecording medium between rollers or heating may be performed throughsandwiching the recording medium using two heating rollers, ifnecessary. For example, the heat generation unit is preferably a halogenlamp heater, a ceramic heater, a nichrome wire, or the like.

A belt base material which constitutes a heating and pressing belt usedin the heating and pressing unit is preferably a seamless electroformednickel, the thickness of the base material is preferably 10 to 100 μm.Further, aluminum, iron, polyethylene, or the like, other than nickelcan be used as the material of belt base material. When silicon resin orfluorocarbon resin is provided, the thickness of layer which is formedby using the resins is preferably 1 to 50 μm, and more preferably 10 to30 μm.

Further, in order to realize the pressure (NIP pressure), elasticmembers such as springs with tensile force are selected and are disposedin both ends of rollers such as a heating and pressing roller so as toobtain the desired NIP pressure in consideration of the NIP gap.

The speed of conveyance of the recording medium when a heating andpressing roller or a heating and pressing belt is used is preferably inthe range of 200 mm/second to 700 mm/second, more preferably 300mm/second to 650 mm/second, and further preferably 400 mm/second to 600mm/second.

—Recording Medium—

The image forming method of the invention is to record an image on therecording medium.

The recording medium is not particularly limited, and general printingpaper including cellulose as a main component such as so-calledhigh-quality paper, coated paper, and art paper may be used. The generalprinting paper including cellulose as a main component absorbs and driesan ink relatively slowly, easily causes coloring material movement aftera droplet is spotted, and allows image quality to easily deteriorate inimage recording by a general inkjet method using an aqueous ink.However, according to the image forming method of the invention,coloring material movement is suppressed, and a high-quality imageexcellent in color density and hue may be recorded.

As the recording medium, a recording medium which is generallycommercially available may be used, and examples include high qualitypaper such as OK Prince High Quality (trade name, manufactured by OjiPaper Co., Ltd.), Shiorai (trade name, manufactured by Nippon PaperIndustries Co., Ltd.), and New NP High Quality (trade name, manufacturedby Nippon Paper Industries Co., Ltd.), fine coated paper such as OK EverLite Coat (trade name, manufactured by Oji Paper Co., Ltd.) and Aurora S(trade name, Nippon Paper Industries Co., Ltd.), light coated paper (A3)such as OK Coat L (trade name, manufactured by Oji Paper Co., Ltd.) andAurora L (trade name, manufactured by Nippon Paper Industries Co.,Ltd.), coated paper (A2, B2) such as OK Top Coat+ (trade name,manufactured by Oji Paper Co., Ltd.) and Aurora Coat (trade name,manufactured by Nippon Paper Industries Co., Ltd.), and an art paper(A1) such as OK Kanefuji+ (trade name, manufactured by Oji Paper Co.,Ltd.) and Tokubishi Art (trade name, manufactured by Nippon PaperIndustries Co., Ltd.). Further, various papers for photography for usein inkjet recording may be used.

Among the above, a recording medium having a water-absorptioncoefficient Ka of 0.05 mL/m²·ms^(1/2) to 0.5 mL/m²·ms^(1/2) ispreferable, a recording medium having a water-absorption coefficient Kaof 0.1 mL/m²·ms^(1/2) to 0.4 mL/m²·ms^(1/2) is more preferable, and arecording medium having a water-absorption coefficient Ka of 0.2mL/m²·ms^(1/2) to 0.3 mL/m²·ms^(1/2) is even more preferable from theviewpoints of the large suppression effect on color material movementand obtaining high quality image having good color density and color huecompared to conventional methods.

The water-absorption coefficient Ka has the same definition as thatdescribed in JAPAN•TAPPI•Paper Pulp Testing Method No 51:2000 (publishedby Japan Technical Association of the Pulp and Paper Industry).Specifically, the absorption coefficient Ka is calculated fromdifference of the transferring amount of water in contact time 100 msand 900 ms by using Automatic Scanning Absorptometer KM500Win (tradename, manufactured by Kumagai Riki Kogyo Co., Ltd.).

Among recording mediums, there is preferred a so-called coated paperused in general offset printing, or the like. The coated paper is acoating layer provided by applying a coating material on the surface ofa high-quality paper, a neutralized paper, or the like which mainly usecellulose and are generally not surface-treated. The coated paper easilycauses problems in quality in gloss or abrasion resistance of the image,or the like in forming an image by a conventional water-based inkjetmethod. In the image forming method of the invention, gloss unevennesscan be suppressed to obtain good image having glossy and abrasionresistance. In particular, the coated paper is preferably used which hasbase paper and a coating layer including kaolin and/or calciumbicarbonate. More specifically, an art paper, a coated paper, alightweight coated paper or a micro coated paper are more preferred.

—Ink Composition—

The ink composition of the invention includes an inorganic silicatecompound and is generally composed by including a further colorant suchas a pigment or dye, and further, is composed by using another componentif necessary. In the invention, the composition which contains pigmentand inorganic silicate compound is preferable. The pigment (hereinafter,referred to as “resin-coated pigment”) is coated with a water insolubleresin including a structural unit having an acidic group.

By using an ink composition constituted by including an inorganicsilicate compound and preferably a resin-coated pigment, deteriorationby erosion of the nozzle plate of the inkjet head is suppressed and isexcellent in the ejection reliability of ink. Further, the abrasionresistance of the formed image is increased.

In case of the inkjet head configuration, in order to precisely formmicro-nozzle (ink ejection port), there is a case where silicon or thelike is added to constitute nozzle plate. In the inkjet head having thesilicon nozzle plate, there is a case where deterioration such as shapedeformation due to erosion of the nozzle plate by contact with ink anddecrease of liquid repellent properties has an impact on ink ejection.The ink composition of the invention can more effectively suppressdeterioration by erosion of the nozzle plate and decrease of liquidrepellent properties when the inkjet head having a nozzle plate formedof silicon, or the like is used.

The ink composition of the invention contains at least one kind ofinorganic silicate compound. The inorganic silicate compound may bewidely selected from silicic acid and silicate; in particular, salt withalkali metal and alkali earth metal of silicic acid such as sodiumsilicate, potassium silicate, calcium silicate, and magnesium silicate,or anhydrous silicic acid (silica) is preferable. An alkali solution ofan alkali metal salt of silicic acid which is referred to as water glassis preferably used as the silicate. The anhydrous silicic acid (silica)is not particularly limited, but colloidal silica is preferably used.

As far as the alkali metal salt of silicic acid is a compound which isconstituted by silicon dioxide and metallic oxide and has watersolubility, it is not particularly limited. The alkali metal salt ofsilicic acid includes alkali metal salt of metasilicic acid, alkalimetal salt of orthosilicic acid, or the like. Further, an ammonium saltof silicic acid including an ammonium salt of metasilicic acid, ammoniumsalt of orthosilicic acid, or the like may be also used. The silicatesalt having water solubility may be used alone or in a combination withtwo or more kinds thereof.

Specifically, the alkali metal salt of silicic acid is preferably atleast one kind of compound represented by the following formula (S).x(M₂O)·y(SiO₂)  Formula (S)

In Formula (S), M represents sodium or potassium, x represents 1 or 2, yrepresents an integer of 1 to 4. The alkali metal salt of silicic acidrepresented by Formula (S) is referred as the alkali metal salt ofsilicic acid when x=1, y=1, alkali metal salt of orthosilicic acid whenx=2, y=1, and both are alkali metal salt of silicic acid having watersolubility.

As the alkali metal salt of silicic acid having water solubility, acommercial compound (for example, water glass), or one obtained bysolving silicic acid and carbonate or hydroxide of alkali metal may beused.

Among them, from the viewpoints of suppressing elution of the portioncontacting the ink composition of the inkjet head (particularly, nozzleplate or ink flow path), and an erosion according to the elution,incorporating at least one kind selected from alkali metal salt ofsilicic acid such as sodium silicate and potassium silicate in the inkcomposition is preferable. The alkali metal salt of silicic acid rendersto obtain good ink dispersibility to the ink composition in comparisonwith salt other than alkali metal, for example ammonium salt of silicicacid (for example, tetramethyl ammonium salt of silicic acid, or thelike). Further, in the case of ammonium salt, or the like, a volatilecompound can be produced in some cases, and thus an alkali metal salt ofsilicic acid is preferable from the viewpoint that over time odors arehardly generated.

Colloidal silica is colloid that comprises fine particles of inorganicoxides including silicon, in which an average particle diameter of thefine particles is several hundred nm or less. Colloidal silica includessilicon dioxide (including hydrates thereof) as a main component and maycontain aluminate as a minor component. Examples of the aluminate, whichmay be contained as a minor component, include sodium aluminate andpotassium aluminate.

Further, inorganic salts such as sodium hydroxide, potassium hydroxide,lithium hydroxide, and ammonium hydroxide or organic salts such astetramethylammonium hydroxide may be contained in the colloidal silica.These inorganic salts and organic salts function, for example, as astabilizer of colloid.

The dispersing medium for colloidal silica is not particularly limitedand may be any of water, an organic solvent, or a mixture of water andan organic solvent. The organic solvent may be a water-soluble organicsolvent or a water-insoluble organic solvent. However, the organicsolvent is preferably a water-soluble organic solvent. Specific examplesthereof include methanol, ethanol, isopropyl alcohol, and n-propanol.

There is no particular limitation on the method for producing colloidalsilica, and colloidal silica can be produced by a generally used method.For example, colloidal silica can be produced through an Aerosilsynthesis by thermal decomposition of silicon tetrachloride, or may beproduced from water glass. Alternatively, colloidal silica can beproduced according to a liquid phase synthesis method includinghydrolysis of an alkoxide (see, for example, “Seni to Kogyo”, vol. 60,No. 7, page 376, 2004), or the like.

There is no particular limitation on the average particle diameter ofthe particles contained in the colloidal silica according to the presentinvention. For example, the average particle diameter may be set from 1nm to 200 nm. The average particle diameter is preferably from 1 nm to100 nm, more preferably from 3 nm to 50 nm, even more preferably from 3nm to 25 nm, and particularly preferably from 5 nm to 20 nm.

When the average particle diameter is 200 nm or less, damages (forexample, shape deformation due to erosion of the nozzle plate, loweringof liquid repellency or the like) caused by ink to the members whichconstruct the inkjet head, such as a substrate, a protective film, aliquid-repellent film, and the like, may be more effectively suppressed.It is thought that, by making the average particle diameter smaller, atotal surface area of particles increases, so that damages to themembers which construct the inkjet head is more effectively suppressed.Moreover, it is preferable that the average particle diameter of theparticles is 200 nm or less, also from the viewpoints of dischargereliability of the ink composition and suppression of the abrasiveeffect caused by the particles.

In the present invention, the average particle diameter of the colloidalsilica is represented by a volume average particle diameter. The volumeaverage particle diameter can be determined according to a generalmethod for dispersed particles such as a light scattering method or alaser diffraction method.

The shape of the colloidal silica is not particularly limited so long asit does not disturb the ejection performance of the ink. For example,the shape may be a spherical shape, a long shape, a needle-like shape,or a shape like a string of beads. Above all, it is preferred that thecolloidal silica is spherical, from the viewpoint of dischargeability ofink.

The colloidal silica, which can be used in the present invention, may beproduced by the production method described above, or may be acommercially available product. Specific examples of the commerciallyavailable product include LUDOX AM, LUDOX AS, LUDOX LS, LUDOX TM, andLUDOX HS (all trade names, manufactured by E.I. Du Pont de Nemours &Co.); SNOWTEX S, SNOWTEX XS, SNOWTEX 20, SNOWTEX 30, SNOWTEX 40, SNOWTEXN, SNOWTEX C, and SNOWTEX 0 (all trade names, manufactured by NissanChemical Industries, Ltd.); SYTON C-30 and SYTON ZOO (all trade names,manufactured by Monsanto Co.); NALCOAG-1060 and NALCOAG-ID21 to 64 (alltrade names, manufactured by Nalco Chem. Co.); METHANOL SOL, IPA SOL,MEK SOL, and TOLUENE SOL (all trade names, manufactured by Fuso ChemicalCo., Ltd.), CATALOID-S, CATALOID-F120, CATALOID SI-350, CATALOID SI-500,CATALOID SI-30, CATALOID S-20L, CATALOID S-20H, CATALOID S-30L, CATALOIDS-30H, CATALOID SI-40, and OSCAL-1432 (isopropyl alcohol sol) (all tradenames, manufactured by JGC Catalysts and Chemicals Ltd.); ADELITE (tradename, manufactured by Asahidenka Co., Ltd.); and, as examples ofcolloidal silica in the shape of a string of beads, SNOWTEX ST-UP,SNOWTEX PS-S, SNOWTEX PS-M, SNOWTEX ST-OUP, SNOWTEX PS-SO, and SNOWTEXPS-MO (all trade names, manufactured by Nissan Chemical Industries,Ltd.). These products are easily available.

The pH of the above commercially available colloidal silica dispersionliquid is often adjusted to pH of acidic or alkaline. This is becausethe region where colloidal silica is stably dispersed exists in anacidic side or alkaline side. In the case of adding a commerciallyavailable colloidal silica dispersion liquid to the ink composition, thepH of the region where the colloidal silica is stably dispersed and thepH of the ink composition should be taken in consideration.

The content of the inorganic silicate compound in the ink composition,while not particularly limited, can be for example 0.0005% by mass to0.5% by mass with respect to the total amount (entire mass) of the inkcomposition. The content of inorganic silicate compound is preferably0.001% by mass to 0.5% by mass with respect to the total amount of theink composition, more preferred is 0.01% by mass to 0.5% by mass of thetotal amount of the ink composition, and particularly preferred is 0.01%by mass to 0.3% by mass of the total amount of the ink composition. Whenthe content of the ink composition is the upper limit or less, theejection properties of the ink composition is more improved, furtherinfluence on inkjet head due to abrasive effect of silica particle canbe suppressed more effectively. Further, when the content of the inkcomposition is the lower limit or higher, shape deformation bydeterioration due to erosion of the nozzle plate and decrease of liquidrepellent property can be suppressed more effectively.

Further, in the ink composition of the invention, it is preferred thatthe content of colloidal silica which has volume average particlediameter of 3 nm to 25 nm is 0.001% by mass to 0.5% by mass with respectto the total amount of the ink composition, from the viewpoints of shapedistortion due to erosion of the nozzle plate, decrease of liquidrepellent property and the ink ejection properties. It is more preferredthat the content of colloidal silica which has volume average particlediameter of 3 nm to 20 nm is 0.01% by mass to 0.5% by mass of the totalamount of the ink composition.

[Colorant]

The ink composition of the invention can contain color elements such aspigments or dyes as colorants. In the invention, it is preferred tocontain at least one kind of pigment which is coated with awater-insoluble resin including a structural unit having an acidicgroup. Thereby, the ink composition of the invention is excellent in inkejection reliability and is excellent in abrasion resistance of theformed image therewith. In this case, a specific form of pigment is notparticularly limited, as long as there is a form where the whole or apart of the surface of the pigment particles is coated with the waterinsoluble resin.

<Pigment>

The pigment used in the exemplary embodiment of the invention is notparticularly limited, and may be appropriately selected according to theintended use. The pigment includes an organic pigment and an inorganicpigment.

Examples of the organic pigment include azo pigments, polycyclicpigments, dye chelates, nitro pigments, nitroso pigments, and anilineblack. Among them, azo pigments and polycyclic pigments are morepreferable.

Examples of the azo pigments include azo lakes, insoluble azo pigments,condensed azo pigments, and chelate azo pigments.

Examples of the polycyclic pigment include phthalocyanine pigments,perylene pigments, perinone pigments, anthraquinone pigments,quinacridone pigments, dioxazine pigments, indigo pigments, thioindigopigments, isoindolinone pigments, and quinophthalone pigments.

Examples of the dye chelates include basic dye chelates and acidic dyechelates.

Examples of the inorganic pigments include titanium oxide, iron oxide,calcium carbonate, barium sulfate, aluminium hydroxide, barium yellow,cadmium red, chrome yellow, and carbon black. Among them, carbon blackis particularly preferable. Carbon black may be produced by a knownmethod such as a contact method, a furnace method, or a thermal method.

These pigments may be used alone or in a combination of two or more ofthem selected from one or more groups above.

(Water-insoluble Resin)

The water-insoluble resin contains at least one structural unit havingan acidic group, and may further contain one or more other structuralunit(s) if necessary. In preferable embodiments, in view of achievingstable presence in the ink composition, reducing adhering andaccumulation of aggregates, and enabling easy removal of adheredaggregates, the water-insoluble resin may preferably contain at leastone hydrophilic structural unit (A) and at least one hydrophobicstructural unit (B). In more preferable embodiments, the acidic groupmay be contained in one of the hydrophilic structural unit (A).

The “water-insoluble polymer” herein refers to a polymer whose dissolvedamount to 100 g of water at 25° C. is 5 g or smaller when the polymer isdissolved in the water. The “dissolved amount” is an amount of (a partof) the water-insoluble polymer dissolved in a solvent (water) whenacidic groups of the water-insoluble polymer are completely neutralizedwith sodium hydroxide.

Hydrophilic Structural Unit

There is no particular limitation to the hydrophilic structural unit inthe water-insoluble polymer as long as it contains at least onehydrophilic functional group. The hydrophilic structural unit maycontain an ionic hydrophilic group or a nonionic hydrophilic group. Inpreferable embodiments, the hydrophilic structural unit may have anacidic group. The hydrophilic structural unit having an acidic group maybe derived from a monomer including an acidic group, or may be astructural unit formed by introducing, by a macromolecular reaction, anacidic group to a structural unit having no acidic group in a polymerchain which has been formed by polymerization.

The acid group is not particularly limited and may include, from theviewpoint of stability of the emulsion state or dispersion state, acarboxy group, a phosphoric acid group, and a sulfonic acid group. Amongthese, a carboxy group is preferable from the viewpoint of dispersionstability in an ink composition.

As a monomer having an acid group (acid group containing monomer), amonomer having an acid group and an ethylenically unsaturated bond ispreferable. Examples of the monomer having an acid group may include anunsaturated carboxylic acid monomer, an unsaturated sulfonic acidmonomer, and an unsaturated phosphoric acid monomer.

Examples of the unsaturated carboxylic monomer may include acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid, citraconic acid, and 2-methacryloyloxymethyl succinic acid.Examples of the unsaturated sulfonic acid monomer may includestyrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid,3-sulfopropyl (meth)acrylate, and bis(3-sulfopropyl) itaconate. Examplesof the unsaturated phosphoric acid monomer may include vinylphosphonicacid, vinyl phosphate, bis(methacryloxyethyl) phosphate,diphenyl-2-acryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethylphosphate, and dibutyl-2-acryloyloxyethyl phosphate.

Among the acid group containing monomers, from the viewpoints ofdispersion stability and ejection stability, an unsaturated carboxylicmonomer is preferable, and acrylic acid and methacrylic acid are morepreferable. Specifically, the structural unit having an acid group ispreferably a structural unit derived from (meth)acrylic acid.

In the water-insoluble resin, either or both of a structural unitderived from acrylic acid and a structural unit derived from methacrylicacid are preferably incorporated.

When the hydrophilic group includes a basic group, examples of the basicgroup include an amino group and an amido group in which a nitrogen atomis unsubstituted.

Examples of the hydrophilic structural unit (A) having a basic groupinclude a structural unit derived from a monomer having a basichydrophilic group. Examples of the monomer having a basic hydrophilicgroup include (meth)acrylate having a basic hydrophilic group,(meth)acrylamide having a basic hydrophilic group, and vinyl monomerssuch as vinyl esters having a basic hydrophilic group.

A monomer which provides the hydrophilic structural unit having a basichydrophilic group may preferably have a functional group which can forma polymer such as an ethylenically unsaturated bond and a basichydrophilic functional group. Such monomer may be selected from knownmonomers, and specific examples thereof which may be preferably usedinclude (meth)acrylamides, aminoethyl (meth)acrylates, and aminopropyl(meth)acrylates.

When the hydrophilic group includes a nonionic hydrophilic group,examples of the nonionic hydrophilic group include a hydroxyl group andalkylene oxides such as polyethylene oxide or polypropylene oxidedescribed below.

Examples of the hydrophilic structural unit (A) having a nonionichydrophilic group include a unit derived from a monomer having anonionic hydrophilic group. Examples of the monomer having a nonionichydrophilic group include (meth)acrylate having a nonionic hydrophilicgroup, (meth)acrylamide having a nonionic hydrophilic group, and vinylmonomers such as vinyl esters having a hydrophilic group.

The monomer that forms the hydrophilic structural unit having a nonionichydrophilic group is preferably a monomer that has a functional groupcapable of forming a polymer such as an ethylenically unsaturated bondand a nonionic hydrophilic functional group, and may be selected fromknown monomers. Preferable specific examples of the monomer may includehydroxylethyl (meth)acrylate, hydroxybutyl (meth)acrylate, and(meth)acrylate that contains an alkyleneoxide polymer.

The hydrophilic structural unit (A) having a nonionic hydrophilic groupmay be formed by polymerization of corresponding monomers, but may beformed by introducing a hydrophilic functional group into a polymerchain after polymerization.

As the hydrophilic structural unit having a nonionic hydrophilic group,a hydrophilic structural unit having an alkylene oxide structure is morepreferable. As the alkylene moiety of the alkylene oxide structure, fromthe viewpoint of hydrophilicity, an alkylene moiety having 1 to 6 carbonatoms is preferable, an alkylene moiety having 2 to 6 carbon atoms ismore preferable, and an alkylene moiety having 2 to 4 carbon atoms isparticularly preferable. The polymerization degree of the alkylene oxidestructure is preferably 1 to 120, more preferably 1 to 60, andparticularly preferably 1 to 30.

In a preferable embodiment, the hydrophilic structural unit having anonionic hydrophilic group is a hydrophilic structural unit havinghydroxy group. The number of hydroxy groups in the structural unit isnot particularly limited and is preferably 1 to 4, more preferably 1 to3, and still more preferably 1 or 2, from the viewpoints of thehydrophilicity of the water-insoluble resin and compatibility with asolvent and other monomers at the time of polymerization.

In the foregoing description, the content of the hydrophilic structuralunit varies, for example, depending on the ratio of the hydrophobicstructural unit (B) described later. For example, when thewater-insoluble resin is composed of acrylic acid and/or methacrylicacid (hydrophilic structural unit (A)) and the hydrophobic structuralunit (B) described later, the content of acrylic acid and/or methacrylicacid is determined by “100−(% by mass of the hydrophobic structuralunit)”.

The hydrophilic structural units (A) may be used alone or as a mixtureof two or more of them.

—Hydrophobic Structural Unit—

In embodiments, the water-insoluble polymer may preferably further haveat least one hydrophobic structural unit (B) other than the structuralunit having an acidic group. There is no particular limitation to thehydrophobic structural unit in the water-insoluble polymer as long as itcontains at least one hydrophobic functional group. In embodiments, thehydrophobic structural unit may preferably include at least onestructural unit having an aromatic ring, and may more preferably includea structural unit represented by the following Formula (1).

In Formula (1), R₁ represents a hydrogen atom or a methyl group. L₁represents an unsubstituted or substituted phenylene group. L₂represents a single bond or a divalent linking group. Ar¹ represents amonovalent group derived from a condensed aromatic ring having 8 or morecarbon atoms, a heterocycle having an aromatic ring condensed therein,or a compound having two or more benzene rings linked to each other.

In Formula (1), R₁ represents a hydrogen atom or a methyl group, andpreferably a methyl group.

L₁ represents an unsubstituted or substituted phenylene group. Anunsubstituted phenylene group is preferable as L₁. L₂ represents asingle bond or a divalent linking group. The divalent linking group ispreferably a linking group having 1 to 30 carbon atoms, more preferablya linking group having 1 to 25 carbon atoms, even more preferably alinking group having 1 to 20 carbon atoms, and particularly preferably alinking group having 1 to 15 carbon atoms. Particularly preferableexamples of the linking group include an alkyleneoxy group having 1 to25 carbon atoms (more preferably 1 to 10 carbon atoms), an imino group(—NH—), a sulfamoyl group, a divalent linking group including analkylene group such as an alkylene group having 1 to 20 carbon atoms(more preferably 1 to 15 carbon atoms) or an ethylene oxide group[—(CH₂CH₂O)_(n)—, n=1 to 6], and a combination of two or more thereof.

Ar¹ represents a monovalent group derived from a condensed aromatic ringhaving 8 or more carbon atoms, a heterocycle having an aromatic ringcondensed therein, or a compound having two or more benzene rings linkedto each other.

The “condensed aromatic ring having 8 or more carbon atoms” may be anaromatic ring having two or more benzene rings condensed therein or anaromatic ring having 8 or more carbon atoms composed of at least onearomatic ring and a ring formed by an alicyclic hydrocarbon condensedwith the aromatic ring. Specific examples include naphthalene,anthracene, fluorene, phenanthrene, and acenaphthene.

The “heterocycle having an aromatic ring condensed therein” is acompound consisting of a heteroatom-free aromatic compound (preferably abenzene ring) and a heteroatom-containing cyclic compound condensed witheach other. The heteroatom-containing cyclic compound is preferably afive- or six-membered ring. The heteroatom is preferably a nitrogenatom, an oxygen atom or a sulfur atom. The heteroatom-containing cycliccompound may have a plurality of heteroatoms. In this case, theheteroatoms may be the same as or different from each other. Specificexamples of the heterocycle having an aromatic ring condensed thereininclude phthalimide, acridone, carbazole, benzoxazole, andbenzothiazole.

Examples of the compound having two or more benzene rings linked to eachother include compounds having two or more benzene rings linked to eachother via a single bond or a linking group having 1 or 2 carbon atoms.

Specific examples of the monovalent group derived from a compound havingtwo or more benzene rings linked to each other include a biphenyl group,a terphenyl group, a diphenylmethyl group, a triphenylmethyl group andthe like.

Specific examples of monomers that forms the structural unit representedby Formula (1) include the following monomers. The present invention isnot limited to these monomers.

M-25/M-27 represents a mixture of monomers M-25 and M-27, each of whichhas the substituent at m- or p-position.

M-28/M-29 represents a mixture of monomers M-28 and M-29, each of whichhas the substituent at m- or p-position.

Ar¹ in the structural unit represented by Formula (1) is preferably amonovalent group derived from acridone or phthalimide from the viewpointof the dispersion stability of the coated pigment, and more preferably amonovalent group derived from acridone.

As the structural unit represented by Formula (1), from the viewpoint ofdispersion stability of the pigment, a structural unit that is specifiedby selecting an unsubstituted phenylene group as L₁, a divalent linkinggroup (preferably methylene) as L₂, and a monovalent group derived fromacridone as Ar¹ is preferable.

The content of the structural unit represented by Formula (1) in thecopolymer is preferably in the range of from 5% by mass to 25% by mass,with respect to the total mass of the copolymer, and more preferably inthe range of from 10% by mass to 18% by mass.

When the content is 5% by mass or more, generation of image defects suchas white spots tends to be suppressed markedly desirably, on the otherhand, when the content is 25% by mass or less, problems of productionsuitability caused by lowering the solubility of the copolymer in apolymerization reaction liquid (for example, methyl ethyl ketone) tendnot to be brought about desirably.

The water-insoluble resin in the preferable exemplary embodiment of theinvention may include a structural unit represented by the followingFormula (2) other than the structural unit represented by Formula (1).

In Formula (2), R² represents a hydrogen atom or a methyl group, andpreferably a methyl group. Ar¹ represents a monovalent group derivedfrom an unsubstituted or substituted aromatic ring (aromatic ringgroup). When the aromatic ring is substituted by a substituent, examplesof the substituent include a halogen atom, an alkyl group, an alkoxygroup, a hydroxy group, a cyano group and, an alkoxycarbonyl group, andthe aromatic ring may form a condensed ring. When the aromatic ringforms a condensed ring, the condensed ring may be, for example, acondensed aromatic ring having 8 or more carbon atoms, or an aromaticring having a heterocycle condensed therein. Further, Ar² may be amonovalent group derived from a compound having two or more aromaticrings linked to each other.

In Formula (2), each of “a condensed aromatic ring having 8 or morecarbon atoms” and “an aromatic ring having a heterocycle condensedtherein” has the same definition as “a condensed aromatic ring having 8or more carbon atoms” and “an aromatic ring having a heterocyclecondensed therein” in Formula (1) respectively. Further, “a monovalentgroup derived from a compound having two or more aromatic rings linkedto each other” in Formula (2) preferably includes “a monovalent groupderived from a compound having two or more aromatic rings linked to eachother” in Formula (1).

The aromatic ring group represented by Ar² is linked via an ester groupand an ethylene oxide chain to the main chain of the water-insolubleresin, and the aromatic ring group is not directly linked to the mainchain, and thus a suitable distance is maintained between thehydrophobic aromatic ring and the hydrophilic structural unit, so thatthe water-insoluble resin interacts readily with, and is adsorbed firmlyonto, a pigment to improve dispersibility.

In particular, the aromatic ring group represented by Ar² is preferablyan unsubstituted phenyl group or an unsubstituted naphthyl group, andparticularly preferably an unsubstituted phenyl group.

n is an average repeating number of the ethyleneoxy units in thewater-insoluble resin used for the resin-coated pigment contained in theaqueous ink composition. n is in the range of 1 to 6, and preferably 1to 2.

Specific examples of monomers that forms the structural unit representedby Formula (2) include the following monomers.

From the viewpoint of dispersion stability, it is particularlypreferable that in the structural unit represented by Formula (2), R² isa methyl group, Ar² is an unsubstituted phenyl group, and n is 1 to 2.

The content of the structural unit of Formula (1) in the water-insolubleresin is preferably in the range of 30% by mass to 70% by mass, and morepreferably in the range of 40% by mass to 50% by mass, based on thetotal mass of the water-insoluble resin. When the content is 30% by massor more, dispersibility is good, and when the content is 70% by mass orless, the adhesion and deposition of the aggregate may be prevented, theremovability of adhered aggregate (maintenance properties) is good, andgeneration of imaging defects such as white spots may be prevented.

The water-insoluble resin in the invention is preferably a resinincluding a hydrophilic structural unit (A) and a hydrophobic structuralunit (B), from the viewpoint of allowing the water-insoluble resin to bestably present in an aqueous ink, to reduce adhesion or deposition ofthe aggregate, and to facilitate removal of the adhered aggregate.Herein, the hydrophobic structural unit (B) preferably includes thestructural unit represented by Formula (1) or Formula (2) above.

The water-insoluble resin of the present invention may further have anadditional hydrophobic structural unit (B′) other than the structuralunit represented by Formula (1) and the structural unit represented byFormula (2). Examples of the hydrophobic structural unit (B′) mayinclude a structural units derived from vinyl monomers such as(meth)acrylates, (meth)acrylamides, styrenes or vinylesters which do notbelong to the hydrophilic structural unit (A) (for example, those havingno hydrophilic functional group), a hydrophobic structural unit havingan aromatic ring that is linked to an atom of the main chain thereofthrough a linking group, and the like. These structural units may beused one kind alone or two or more kinds in combination.

Examples of the (meth)acrylates include methyl (meth)acrylate, ethyl(meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and hexyl(meth)acrylate. Among them, methyl (meth)acrylate, ethyl (meth)acrylate,and butyl (meth)acrylate are preferable, and methyl (meth)acrylate andethyl (meth)acrylate are particularly preferable.

Examples of the (meth)acrylamides include N-cyclohexyl (meth)acrylamide,N-(2-methoxyethyl) (meth)acrylamide, N,N-diallyl (meth)acrylamide, andN-allyl (meth)acrylamide.

Examples of the styrenes include styrene, methylstyrene,dimethylstyrene, trimethylstyrene, ethylstyrene, isopropylstyrene,n-butylstyrene, tert-butylstyrene, methoxystyrene, butoxystyrene,acetoxystyrene, chlorostyrene, dichlorostyrene, bromostyrene,chloromethylstyrene, hydroxystyrene protected by a group removable withan acidic substance (for example, t-Boc), methyl vinyl benzoate,α-methylstyrene, and vinylnaphthalene. Among them, styrene andα-methylstyrene are preferable.

Examples of the vinyl esters include vinyl acetate, vinyl chloroacetate,vinyl propionate, vinyl butyrate, vinyl methoxyacetate, and vinylbenzoate. Among them, vinyl acetate is preferable.

The above-described “hydrophobic structural unit containing an aromaticring that is linked to an atom in the main chain via a linking group” ispreferably a structural unit wherein the proportion of the aromatic ringlinked to an atom in the main chain of the copolymer via a linking groupis from 15% by mass to 27% by mass, more preferably from 15% by mass to25% by mass, and even more preferably from 15% by mass to 20% by masswith respect to the copolymer.

The aromatic ring is linked to the atom in the main chain of thecopolymer not directly but via a linking group. Therefore, an adequatedistance is kept between the hydrophobic aromatic ring and thehydrophilic structural unit, so that the copolymer readily interactswith the pigment and is firmly adsorbed thereon, thus improving thedispersibility of the pigment.

The “hydrophobic structural unit containing an aromatic ring that islinked to an atom in the main chain via a linking group” is preferably astructural unit represented by the following Formula (3) (excluding thestructural unit represented by Formula (1) and the structural unitrepresented by Formula (2)).

In Formula (3), R¹¹ represents a hydrogen atom, a methyl group, or ahalogen atom. L¹¹ represents *—COO—, *—OCO—, *—CONR¹²—, or *—O—, and R¹²represents a hydrogen atom or an alkyl group having 1 to 10 carbonatoms. In the group represented by L¹¹, an asterisk (*) denotes a bondconnected to the main chain.

L¹² represents a single bond or a divalent linking group having 1 to 30carbon atoms. When L¹² is a divalent linking group, it is preferably alinking group having 1 to 25 carbon atoms, more preferably a linkinggroup having 1 to 20 carbon atoms, and even more preferably a linkinggroup having 1 to 15 carbon atoms.

Among them, particularly preferable examples include an alkyleneoxygroup having 1 to 25 (more preferably 1 to 10 carbon atoms) carbonatoms, an imino group (—NH—), a sulfamoyl group, and divalent linkinggroups containing an alkylene group, such as an alkylene group having 1to 20 carbon atoms (more preferably 1 to 15 carbon atoms) or an ethyleneoxide group [—(CH₂CH₂O)_(n)—, n=1 to 6], and combinations of two or moreof these groups.

In Formula (3), Ar¹¹ represents a monovalent group derived from anaromatic ring.

The aromatic ring group which derives the monovalent group representedby Ar¹¹ is not particularly limited, and examples of the aromatic ringinclude a benzene ring, a condensed aromatic ring having eight or morecarbon atoms, an aromatic ring condensed with a heterocycle, and acompound having two or more benzene rings linked to each other. Thedetails about the condensed aromatic ring having eight or more carbonatoms, the aromatic ring condensed with a heterocycle, and a compoundhaving two or more benzene rings linked to each other have beendescribed above.

Specific examples of a monomer capable of forming the “hydrophobicstructural unit containing an aromatic ring that is linked to an atom inthe main chain via a linking group” are shown below. However, theinvention is not limited to the following specific examples.

The water-insoluble resin of the present invention is, among the above,preferably characterized in that the hydrophilic structural unit (A) is(meth)acrylic acid and the hydrophobic structural unit (B) is at leastone kind selected from (i) a structural unit represented by Formula (1)(preferably a structural unit derived from the foregoing M-25/M-27 orM-28/M-29), (ii) a structural unit represented by Formula (2)(preferably a structural unit derived from phenoxyethyl (meth)acrylate),and (iii) a hydrophobic structural unit (B′) other than the foregoingstructural units (preferably a structural unit derived from methyl(meth)acrylate, ethyl (meth)acrylate, or benzyl methacrylate).

Furthermore, the water-insoluble resin of the present invention ispreferably characterized in that the hydrophilic structural unit (A) is(meth)acrylic acid and the hydrophobic structural unit (B) contains atleast one kind of the above (i) and (ii).

Particularly, the water-insoluble resin of the present invention ispreferably characterized in that the hydrophilic structural unit (A) is(meth)acrylic acid and the hydrophobic structural unit (B) contains atleast one kind of the above (i) and (ii) and further contains (iii).

In the water-insoluble resin in the invention, although the ratio of thehydrophilic structural unit (A) to the hydrophobic structural unit (B)(including the structural unit represented by Formula (2), thestructural unit represented by Formula (1) and the other hydrophobicstructural units (B′) depends on the degrees of the hydrophilicity andhydrophobicity of these components, the content of the hydrophilicstructural units (A) in the water-insoluble resin is preferably 15% bymass or less. The content of the hydrophobic structural units (B) ispreferably more than 80% by mass, and more preferably 85% by mass ormore with respect to the total mass of the water-insoluble resin.

When the content of the hydrophilic structural unit (A) is 15% by massor less, the amount of the component that dissolves itself in theaqueous medium is decreased, which results in the improvement of pigmentproperties such as dispersibility, whereby good ink ejection propertiesare achieved during inkjet recording.

The content ratio of the hydrophilic structural unit (A) is preferablymore than 0% by mass but 15% by mass or less, more preferably from 2% bymass to 15% by mass, even more preferably from 5% by mass to 15% bymass, and particularly preferably from 8% by mass to 12% by mass withrespect to the total mass of the water-insoluble resin.

In the invention, the acid value of the water-insoluble resin ispreferably from 30 mgKOH/g to 100 mgKOH/g, more preferably from 30mgKOH/g to 85 mgKOH/g, and particularly preferably from 50 mgKOH/g to 85mgKOH/g from the viewpoints of pigment dispersibility and storagestability.

The acid value is defined as the mass (mg) of KOH necessary forcompletely neutralizing 1 g of the water-insoluble resin, and measuredby the method described in Japanese Industrial Standard (JIS K0070,1992), the disclosure of which is incorporated by reference herein.

The weight average molecular weight (Mw) of the water-insoluble resin inthe invention is preferably 30,000 or more, more preferably from 30,000to 150,000, even more preferably from 30,000 to 100,000, andparticularly preferably from 30,000 to 80,000. If the molecular weightis 30000 or more, the water-insoluble resin may provide a good stericrepulsion effect as a dispersant, and is readily adsorbed on the pigmentowing to the steric effect.

The number average molecular weight (Mn) of the water-insoluble resin ispreferably about 1,000 to about 100,000, and particularly preferablyabout 3,000 to about 50,000. When the number average molecular weight iswithin the above-described range, the water-insoluble resin may serve asa coating on the pigment or a coating of the ink composition. Thewater-insoluble resin in the invention is preferably used in the form ofan alkali metal salt or an organic amine salt.

The molecular weight distribution of the water-insoluble resin in theinvention (weight average molecular weight/number average molecularweight) is preferably from 1 to 6, and more preferably from 1 to 4. Whenthe molecular weight distribution is within the above-described range,the resulting ink has improved dispersion stability and ejectionstability.

The number average molecular weight and the weight average molecularweight are measured by the differential refractometer detection with THFas a solvent in a GPC analyzer using columns TSKgel Super HZM-H, TSKgelSuper HZ4000 and TSKgel Super HZ2000 (trade name; all manufactured byTosoh Corporation), and is obtained in terms of polystyrene used as areference material.

The water-insoluble resin in the invention may be synthesized by anypolymerization method, for example, solution polymerization,precipitation polymerization, suspension polymerization, bulkpolymerization, or emulsion polymerization. The polymerization reactionmay be carried out under a known system, such as a batch,semi-continuous, or continuous system. Initiation of the polymerizationmay be carried out with a radical initiator, or photoirradiation orradiation-irradiation. These methods of polymerization and initiation ofpolymerization are described in, for example, “Kobunshi Gosei Hoho” byTeiji Tsuruta, Revised Edition (published by Nikkan Kogyo Shimbun, Ltd.,1971) and “Kobunshi Gosei no Jikkenho” by Takayuki Ohtu and MasaetuKinoshita (published by Kagaku-Dojin Publishing Company Inc., 1972)pages 124 to 154.

Among these polymerization methods, a solution polymerization methodusing a radical initiator is preferable. Examples of the solvent used inthe solution polymerization method include various organic solvents suchas ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, methylisobutyl ketone, cyclohexanone, tetrahydrofuran, dioxane,N,N-dimethylformamide, N,N-dimethylacetamide, benzene, toluene,acetonitrile, methylene chloride, chloroform, dichloroethane, methanol,ethanol, 1-propanol, 2-propanol, and 1-butanol. These solvents may beused alone or in a combination of two or more of them, or may be mixedwith water as a mixed solution. The polymerization temperature should bechosen in consideration of the molecular weight of the intended polymerand the type of the initiator, and is usually from 0° C. to 100° C., andis preferably from 50° C. to 100° C. The reaction pressure may beappropriately selected, and is usually from 1 kg/cm² to 100 kg/cm², andparticularly preferably from about 1 kg/cm² to about 30 kg/cm². Thereaction period may be about 5 hours to about 30 hours. The resultingresin may be subjected to purification treatment such asreprecipitation.

Specific examples of preferable water-insoluble resins of the inventionare shown below. The invention is not limited to these examples. In thefollowing Formula, a, b and c each independently represent the contentof the correspondent structural unit % by mass in the polymer.

R¹¹ n R²¹ R³¹ R³² a b c Mw B-1 CH₃ 1 CH₃ CH₃ —CH₃ 60 9 31 35500 B-2 H 1H H —CH₂CH₃ 69 10 21 41200 B-3 CH₃ 2 CH₃ CH₃ —CH₃ 70 11 19 68000 B-4 CH₃4 CH₃ CH₃ —CH(CH₃)CH₃ 70 7 23 72000 B-5 H 5 H H —CH₃ 70 10 20 86000 B-6H 5 H H —CH₂CH(CH₃)CH₃ 70 2 28 42000 B-7 CH₃ 1 CH₃ CH₃ —CH₂CH₃ 50 11 3944500 B-8 CH₃ 1 CH₃ CH₃ —CH₂CH₃ 50 10 40 51200 B-9 H 1 H H —CH₂CH₃ 45 1144 48900 B-10 H 1 CH₃ CH₃ —CH₂CH₃ 45 12 43 43600

Mw B-11

72400 B-12

33800 B-13

39200

The weight ratio (p:r) between the pigment (p) and the water-insolubleresin (r) in the invention is preferably from 100:25 to 100:140, andmore preferably from 100:25 to 100:50. When the proportion of thewater-insoluble resin is 25 or more, dispersion stability and abrasionresistance tend to improve, and when 140 or less, dispersion stabilitytends to improve.

The resin-coated pigment (capsulated pigment) in the invention may beproduced using a water-insoluble resin and a pigment by a known physicalor chemical method such as that described in JP-A Nos. 9-151342,10-140065, 11-209672, 11-172180, 10-25440, and 11-43636. Specificexamples of the method include the phase inversion method and acidprecipitation method described in JP-A Nos. 9-151342 and 10-140065. Ofthese methods, the phase inversion method is preferable from theviewpoint of dispersion stability.

Basically, the phase inversion method is a self dispersion (phaseinversion emulsification) method comprising dispersing in water a mixedmelt of a pigment and a resin having self dispersibility or solubility.The mixed melt may contain a curing agent or a polymer compound. Themixed melt refers to a state where undissolved components are mixedand/or a state where dissolved components are mixed. Details about the“phase inversion method” are described in JP-A No. 10-140065.

In the ink composition in the invention, the resin-coated pigment ispreferably prepared using the water-insoluble resin through apreparation method of preparing a dispersion of the resin-coated pigmentincluding, for example, the following steps (1) and (2). The inkcomposition of the invention may be prepared by preparing a dispersionof the resin-coated pigment in accordance with the above-describedpreparation method, followed by preparing an ink composition from theobtained dispersion of the resin-coated pigment, water, and ahydrophilic organic solvent.

Step (1): a mixture containing a water-insoluble resin including thestructural unit having an acidic group, an organic solvent, aneutralizing agent, a pigment, and water is dispersed with a stirrer orthe like to obtain a dispersion.

Step (2): at least a part of the organic solvent is removed from thedispersion.

The stirring method is not particularly limited, and may use a commonmixing stirrer or, if necessary, a disperser such as an ultrasonicdisperser, a high-pressure homogenizer, or a bead mill.

Examples of the organic solvent preferable herein include alcoholsolvents, ketone solvents, and ether solvents. Examples of the alcoholsolvents include isopropyl alcohol, n-butanol, t-butanol, and ethanol.Examples of the ketone solvents include acetone, methyl ethyl ketone,diethyl ketone, and methyl isobutyl ketone. Examples of the ethersolvents include dibutyl ether and dioxane. Among these solvents, ketonesolvents such as methyl ethyl ketone and alcohol solvents such asisopropyl alcohol are preferable, and methyl ethyl ketone is morepreferable.

The neutralizing agent may be preferably used in the process (1) forneutralizing a part or all of the acidic groups so that thewater-insoluble resin can form a stable emulsion or dispersion in water.Examples of the neutralizing agent include alcohol amines (such asdiethanolamine, triethanolamine, and 2-amino-2-ethyl-1,3-propanediol),alkali metal hydroxides (such as lithium hydroxide, sodium hydroxide,and potassium hydroxide), ammonium hydroxide (such as ammonium hydroxideand quaternary ammonium hydroxide), phosphonium hydroxides, and alkalimetal carbonates. Among them, sodium hydroxide and potassium hydroxidemay be preferably used.

The water-insoluble resin may have a neutralization degree of from 70%to 95%. When the neutralization degree is 70% or higher, generation ofwhite spots in an image formed from the ink may be suppressed. When theneutralization degree is 95% or lower, abrasion resistance of an imageformed from the ink may be improved.

The neutralization degree may be preferably from 70% to 90%, and morepreferably from 75% to 90%. By adjusting the neutralization degreewithin the above range, generation of white spots in an image formedfrom the ink may be effectively suppressed, and abrasion resistance ofan image formed from the ink may be effectively improved.

The “neutralization degree” herein referred is a ratio (%) of anequivalent of a neutralizer with respect to one equivalent of the acidgroup. Namely, the neutralization degree of the water-insoluble resin isdefined as a ratio of the total equivalence of the neutralizer to thetotal equivalence of the acid group contained in the water-insolubleresin, and is obtained in accordance with the following equality.

Neutralization degree of water-insoluble resin=(total equivalence ofneutralizer)/total equivalence of acid group in water-insolubleresin)×100(%)

In the process (2), the organic solvent is evaporated from thedispersion prepared in the process (1) by a common procedure such asvacuum distillation to convert the phase into a water system, therebyobtaining a dispersion of resin-coated pigment particles, the particlesurface of the pigment being coated with the water-insoluble resin. Theobtained dispersion is substantially free from the organic solvent. Theamount of the organic solvent may be preferably 0.2% by mass or less,and more preferably 0.1% by mass or less.

More specifically, the method for forming the dispersion of thewater-insoluble resin may include: (1) mixing an acid group-containingwater-insoluble resin or its solution in an organic solvent with a basiccompound (neutralizing agent), thereby carrying out neutralization; (2)mixing the obtained mixed solution with a pigment to make a suspension,and then subjecting the suspension to dispersing by using a disperser orthe like to obtain a pigment dispersion; and (3) removing the organicsolvent by, for example, distillation, thereby coating the pigment witha water-insoluble resin having a structural unit having an acid group,and dispersing the coated pigment particles in an aqueous medium toprovide an aqueous dispersion.

The method is further specifically described in JP-A Nos. 11-209672 and11-172180.

The dispersing may be carried out by using, for example, a ball mill, aroll mill, a bead mill, a high-pressure homogenizer, a high-speedstirring disperser, or an ultrasonic homogenizer.

The average particle diameter of the pigment covered with thewater-insoluble resin may be preferably in the range of 10 nm to 200 nm,more preferably in the range of 10 nm to 150 nm, and even morepreferably in the range of 10 nm to 100 nm. When the average particlediameter is 200 nm or less, the color reproducibility and dottingproperty of the ink under inkjet recording system may become favorable.When the average particle diameter is 10 nm or more, light fastness maybecome favorable.

There is no particular limitation to the particle size distribution ofthe pigment covered with the water-insoluble resin. The polymerparticles may have either a broad particle size distribution or amonodisperse particle size distribution. Two or more colored particles,each of which having a monodisperse particle size distribution, may beused in combination as a mixture.

The average particle diameter and the particle size distribution of thepigment covered with the water-insoluble resin may be measured by, forexample, the dynamic light scattering method.

The pigment covered with the water-insoluble resin may be used singly orin a combination of two or more thereof.

From the viewpoint of the density of an image formed from the inkcomposition, the content of the pigment covered with the water-insolubleresin in the ink composition may be preferably from 0.1% by mass to 25%by mass, more preferably from 1% by mass to 20% by mass, even morepreferably from 1.5% by mass to 15% by mass, and further preferably from1.5% by mass to 10% by mass, with respect to the total amount of the inkcomposition.

The ratio of the content of colloidal silica to the content of thewater-insoluble resin (colloidal silica/water-insoluble resin) in theink composition may be preferably from 0.0001 to 0.5, more preferablyfrom 0.0001 to 0.3, and even more preferably from 0.001 to 0.05, interms of mass from the viewpoints of suppression of shape deformationdue to erosion of the nozzle plate and suppression of deterioration ofthe liquid-repellency of the inkjet head member.

In embodiments which may be preferable in view of ink ejectionreliability, abrasion resistance of an image formed from the inkcomposition, suppression of shape deformation due to erosion of thenozzle plate and suppression of deterioration of the liquid-repellencyof the inkjet head member, the water-insoluble resin may have an acidvalue of from 50 mgKOH/g to 90 mgKOH/g, the colloidal silica may have avolume-average particle diameter of from 3 nm to 50 nm, and the ratio ofthe content of colloidal silica to the content of the water-insolubleresin (colloidal silica/water-insoluble resin) may be from 0.0001 to0.3; and in more preferable embodiments, in the ink composition, thewater-insoluble resin may have an acid value of from 55 mgKOH/g to 80mgKOH/g, the colloidal silica may have a volume-average particlediameter of from 3 nm to 25 nm, and the ratio of the content ofcolloidal silica to the content of the water-insoluble resin (colloidalsilica/water-insoluble resin) may be from 0.001 to 0.05.

[Hydrophilic Organic Solvent]

The ink composition of the invention preferably includes a water-basedmedium. The water-based medium contains at least water as a solvent, butpreferably contains water and at least one hydrophilic organic solvent.The hydrophilic organic solvent is used for the purpose of ananti-drying agent, a wetting agent, or a penetration promoting agent.

An anti-drying agent or a wetting agent is used for the purpose ofpreventing the clogging caused as the ink for inkjet recording dries upat the ink spray orifice of a nozzle. The anti-drying agent or wettingagent is preferably a hydrophilic organic solvent having a lower vaporpressure than water.

Furthermore, for the purpose of making the ink composition penetrateeasily into paper, a hydrophilic organic solvent is suitably used as apenetration promoting agent.

The ink composition of the invention preferably includes at least onetype of a first hydrophilic organic solvent having an I/O value of from0.70 to less than 1.0. When the I/O value of the first hydrophilicorganic solvent is less than 1.00, compatibility with theself-dispersing polymer particles is enhanced, the fixability of theimages formed is more effectively enhanced, and the abrasion resistanceof the images is further enhanced. When the I/O value of the firsthydrophilic organic solvent is 0.70 or more, the stability of the inkcomposition is enhanced.

The I/O value of the hydrophilic organic solvent has the same definitionas that in the self-dispersing polymer which is described below, and iscalculated in a manner substantially similar to that in the calculationof the I/O value for the self-dispersing polymer.

It is preferable that the ink composition of the invention furtherincludes at least one of a second hydrophilic organic solvent having anI/O value of 1.00 to 1.50, in addition to the first hydrophilic organicsolvent. When the I/O value of the second hydrophilic organic solvent is1.00 or more, the stability of the ink composition is more effectivelyenhanced. When the I/O value of the second hydrophilic organic solventis 1.50 or less, deterioration of the fixation properties of the formedimages can be suppressed.

Specific examples of the first hydrophilic organic solvent having an I/Ovalue of 0.70 or more and less than 1.00 include glycol ethers.Propylene glycol ether or ethylene glycol ether is preferable, andpropylene glycol ether is more preferable. Specific examples includetriprolene glycol monomethyl ether (I/O value: 0.80), triprolene glycolmonoethyl ether (I/O value: 0.73), triprolene glycol monobutyl ether(I/O value: 0.61), diprolene glycol monoethyl ether (I/O value: 0.78),diprolene glycol monobutyl ether (I/O value: 0.70), and prolene glycolmonobutyl ether (I/O value: 0.88).

Among these, triprolene glycol monomethyl ether (I/O value: 0.80) ispreferable from the viewpoints of image fixability and ink stability.

Specific examples of the second hydrophilic organic solvent having anI/O value of 1.0 to 1.5, include propylene glycol monomethyl ether (I/Ovalue: 1.50), propylene glycol monoethyl ether (I/O value: 1.20),diethylene glycol monobutyl ether (I/O value: 1.40), triethylene glycolmonobutyl ether (I/O value: 1.20), 2,2-diethyl-1,3-propanediol (I/Ovalue: 1.43), 2-methyl-2-propyl-1,3-propanediol (I/O value: 1.43),2,4-dimethyl-2,4-pentanediol (I/O value: 1.43),2,5-dimethyl-2,5-hexanediol (I/O value: 1.25), tripropylene glycol (I/Ovalue: 1.33), SANNIX GP250 (trade name, I/O value: 1.30, manufactured bySanyo Chemical Industries, Ltd.), and the like. Among them, SANNIX GP250is preferable from the viewpoints of image fix properties and inkstability.

The content of the first hydrophilic organic solvent in the inkcomposition for inkjet recording of the invention is preferably 0.1% bymass to 20% by mass, more preferably 1% by mass to 16% by mass, andfurther preferably 2% by mass to 12% by mass, from the viewpoints ofimage fix properties and ink stability.

Furthermore, it is preferable that the ink composition includes, as thefirst hydrophilic organic solvent, a hydrophilic organic solvent whoseI/O value is selected from the range of 0.70 or more and less than 1.00,in an amount of 1 to 16% by mass, and it is more preferable that the inkcomposition includes a hydrophilic organic solvent whose I/O value isselected from the range of 0.70 or more and less than 0.90, in an amountof 2% by mass to 12% by mass.

The content of the second hydrophilic organic solvent in the inkcomposition for inkjet recording of the invention is preferably 0.1% bymass to 20% by mass, more preferably 1% by mass to 16% by mass, andfurther preferably 2% by mass to 12% by mass, from the viewpoints ofimage fix properties and ink stability.

Furthermore, it is preferable that the ink composition includes, as thesecond hydrophilic organic solvent, a hydrophilic organic solvent whoseI/O value is selected from the range of 1.00 to 1.50, in an amount of 1%by mass to 16% by mass, and it is more preferable that the inkcomposition includes a hydrophilic organic solvent whose I/O value isselected from the range of 1.20 to 1.40, in an amount of 2% by mass to12% by mass.

Furthermore, the content ratio of the second hydrophilic organic solventto the first hydrophilic organic solvent in the ink composition forinkjet recording of the invention (second hydrophilic organicsolvent/first hydrophilic organic solvent) is preferably 1/10 to 10/1,more preferably 1/4 to 4/1, and further preferably 1/2 to 2/1, from theviewpoints of image fix properties and ink stability.

The ink composition of the invention may further include anotherhydrophilic organic solvent, in addition to the first hydrophilicorganic solvent and the second hydrophilic organic solvent. As for theother hydrophilic organic solvent, polyhydric alcohols are useful forthe purpose of functioning as an anti-drying agent or a wetting agent,and examples include glycerin (I/O value: 5.00), ethylene glycol (I/Ovalue: 2.00), diethylene glycol (I/O value: 5.00), triethylene glycol(I/O value: 3.43), propylene glycol (I/O value: 2.50), dipropyleneglycol (I/O value: 2.00), 1,3-butanediol (I/O value: 2.50),2,3-butanediol (I/O value: 2.50), 1,4-butanediol (I/O value: 2.50),3-methyl-1,3-butanediol (I/O value: 2.00), 1,5-pentanediol (I/O value:2.00), tetraethylene glycol (I/O value: 2.91), 1,6-hexanediol (I/Ovalue: 1.67), 2-methyl-2,4-pentanediol (I/O value: 1.67), polyethyleneglycol (I/O value depends on the number of repetition of the ethylenechain), 1,2,4-butanetriol (I/O value: 3.75), 1,2,6-hexanetriol (I/Ovalue: 2.50), and the like. These may be used individually, or may beused in combination of two or more types.

For the purpose of functioning as a permeation agent, a polyol compoundis preferable, and preferable examples of aliphatic diol include2-ethyl-2-methyl-1,3-propanediol (I/O value: 1.67),3,3-dimethyl-1,2-butanediol (I/O value: 1.67), 5-hexene-1,2-diol,2-ethyl-1,3-hexanediol (I/O value: 2.00), and2,2,4-trimethyl-1,3-pentanediol (I/O value: 1.88).

The content of the other hydrophilic organic solvent may be, forexample, 16% by mass or less, and is preferably 12% by mass or less, andmore preferably 8% by mass or less.

The hydrophilic organic solvent in the ink composition of the inventionmay be used individually, or may be used as mixtures of two or moretypes. The content of the hydrophilic organic solvent is preferably 1%by mass to 60% by mass, more preferably 5% by mass to 40% by mass, andparticularly preferably 10% by mass to 30% by mass, from the viewpointsof stability and ejection properties.

The amount of addition of water used in the invention is notparticularly limited, but the amount is preferably 10% by mass to 99% bymass, more preferably 30% by mass to 80% by mass, and further preferably50% by mass to 70% by mass, in the ink composition, from the viewpointsof securing stability and ejection reliability.

(Resin Particles)

An ink composition according to the invention preferably includes atleast one kind of resin particles from viewpoints of fixability ofimages formed, abrasion resistance of the images, and aggregationproperty of the ink composition. Further, the resin particles arepreferably particles of self-dispersing polymers. Further, an inkcomposition including the resin particles in combination with thesilicate compound exhibits a large suppression effect on a shapevariation of the nozzle plate by corrosion and deterioration of liquidrepellent property of the nozzle plate, although an ink compositionincluding the resin particles is easy to corrode a silicone part of thenozzle plate.

The self-dispersing polymer according to the invention means awater-insoluble polymer which can be in a dispersed state in an aqueousmedium due to the functional group (particularly, an acidic group or asalt thereof) of the polymer itself when brought to a dispersed state byan phase inversion emulsification method in the absence of a surfactant.

Here, the term dispersed state includes both an emulsified state(emulsion) in which a water-insoluble polymer is dispersed in an aqueousmedium in the liquid state, and a dispersed state (suspension) in whicha water-insoluble polymer is dispersed in an aqueous medium in the solidstate.

In regard to the self-dispersing polymer according to the invention, itis preferable that the water-insoluble polymer is a self-dispersingpolymer capable of being in a dispersed state in the solid state, fromthe viewpoint of ink fixation properties obtainable when incorporated inan ink composition.

The method for preparing the emulsified or dispersed state of theself-dispersing polymer, that is, an aqueous dispersion of theself-dispersing polymer, may be a phase inversion emulsification method.The phase inversion emulsification method may be, for example, a methodof dissolving or dispersing the self-dispersing polymer into a solvent(for example, a hydrophilic organic solvent or the like), subsequentlyintroducing the solution or dispersion directly into water withoutadding a surfactant, mixing under stirring the system while asalt-producing group (for example, an acidic group) carried by theself-dispersing polymer is neutralized, removing the solvent, and thenobtaining an aqueous dispersion that has been brought to an emulsifiedor dispersed state.

A stable emulsified or dispersed state for the self-dispersing polymerof the invention means that even when a solution prepared by dissolving30 g of a water-insoluble polymer in 70 g of an organic solvent (forexample, methyl ethyl ketone), a neutralizing agent capable ofneutralizing 100% of the salt-producing group of the water-insolublepolymer (if the salt-producing group is anionic, sodium hydroxide, andif the salt-producing group is cationic, acetic acid), and 200 g ofwater are mixed and stirred (apparatus: a stirring apparatus equippedwith a stirring blade, speed of rotation 200 rpm, for 30 minutes, 25°C.), and then the organic solvent is removed from the liquid mixture,the emulsified or dispersed state remains stable for at least one weekat 25° C., so that the generation of precipitates cannot be verified byvisual inspection.

The stability of the emulsified or dispersed state for theself-dispersing polymer can be confirmed by a precipitation accelerationtest based on centrifugation. The stability obtained by a precipitationacceleration test based on centrifugation can be evaluated by, forexample, adjusting the aqueous dispersion of the polymer particlesobtained by the method described above to a solids concentration of 25%by mass, subsequently centrifuging the dispersion for one hour at 12,000rpm, and measuring the solids concentration of the supernatant obtainedafter centrifugation.

When the ratio of the solids concentration after centrifugation to thesolids concentration before centrifugation is large (a value close to1), it means that precipitation of the polymer particles resulting fromcentrifugation does not occur, that is, the aqueous dispersion of thepolymer particles is more stable. According to the present invention,the ratio of the solids concentration before and after centrifugation ispreferably 0.8 or greater, more preferably 0.9 or greater, andparticularly preferably 0.95 or greater.

Further, the water-insoluble polymer means a polymer showing an amountof dissolution of 10 g or less when the polymer is dried at 105° C. for2 hr and then dissolved in 100 g of water at 25° C. The amount ofdissolution is, preferably, 5 g or less and, more preferably, 1 g orless. The amount of dissolution is the amount of dissolution when thepolymer is neutralized with sodium hydroxide or acetic acid to 100% inaccordance with the kind of the salt-forming group of thewater-insoluble polymer.

The self-dispersing polymer according to the invention is such that thecontent of the water-soluble component exhibiting water-solubility whenbrought to a dispersed state is preferably 10% by mass or less, morepreferably 8% by mass or less, and even more preferably 6% by mass orless. When the water-soluble component is 10% by mass or less, swellingof the polymer particles or fusion of the polymer particles iseffectively suppressed, and a more stable dispersed state can bemaintained. Viscosity increase of the ink composition can also besuppressed, and the ejection stability becomes better when, for example,the ink composition is used for an inkjet recording method.

Here, the water-soluble component means a compound contained in theself-dispersing polymer, where the compound dissolves in water when theself-dispersing polymer is brought to a dispersed state. Thewater-soluble component is a water-soluble compound that isside-produced or incorporated during the production of theself-dispersing polymer.

The self-dispersing polymer according to the invention includes at leastone hydrophilic constituent unit derived from a hydrophilic monomer, andat least one hydrophobic constituent unit derived from a hydrophobicmonomer. The main chain skeleton of the self-dispersing polymer is notparticularly limited, but from the viewpoint of the dispersion stabilityof the polymer particles, the main chain skeleton is preferably a vinylpolymer, and preferably a (meth)acrylic polymer. Here, the (meth)acrylicpolymer means a polymer including at least one of a constituent unitderived from a methacrylic acid derivative and a constituent unitderived from an acrylic acid derivative.

—Hydrophilic Constituent Unit—

The hydrophilic constituent unit in the self-dispersing polymer is notparticularly limited so long as it is derived from a hydrophilicgroup-containing monomer and it may be either a unit derived from onehydrophilic group-containing monomer (hydrophilic monomer) or a unitderived from two or more hydrophilic group-containing monomers. Thehydrophilic group is not particularly limited and it may be either adissociative group or a nonionic hydrophilic group.

The hydrophilic group is preferably a dissociative group from theviewpoints of promoting the self-dispersibility and stability of theformed emulsified or dispersed state and, more preferably, an anionicdissociative group. Examples of the dissociative group include a carboxygroup, a phosphoric acid group, and a sulfonic acid group and, amongthem, a carboxy group is preferred from the viewpoint of the fixingproperty when used in the ink composition.

The hydrophilic group-containing monomer is preferably a dissociativegroup-containing monomer and, preferably, a dissociativegroup-containing monomer having a dissociative group and anethylenically unsaturated bond from the viewpoint ofself-dispersibility.

Examples of the dissociative group-containing monomer include anunsaturated carboxylic acid monomer, an unsaturated sulfonic acidmonomer, and an unsaturated phosphoric acid monomer.

Specific examples of the unsaturated carboxylic acid monomer includeacrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleicacid, fumaric acid, citraconic acid, and 2-(methacryloyloxy) methylsuccinicate, etc. Specific examples of the unsaturated sulfonic acidmonomer include styrenesulfonic acid,2-acrylamido-2-methylpropanesulfonic acid, 3-sulfopropyl (meth)acrylate,and bis(3-sulfopropyl) itaconate. Specific examples of the unsaturatedphosphoric acid monomer include vinylphosphonic acid, vinylphosphate,bis(methacryloyloxyethyl) phosphate, diphenyl-2-acryloyloxyethylphosphate, diphenyl-2-methacryloyloxyethyl phosphate, anddibutyl-2-acryloyloxyethyl phosphate.

Among the dissociative group-containing monomers, an unsaturatedcarboxylic acid monomer is preferred and, at least one kind of acrylicacid and methacrylic acid is more preferred from the viewpoints of thedispersion stability and ejection stability.

Examples of the monomer having a nonionic hydrophilic group includeethylenically unsaturated monomers containing a (poly)ethyleneoxy groupor a polypropyleneoxy group, such as 2-methoxyethyl acrylate,2-(2-methoxyethoxy)ethyl acrylate, 2-(2-methoxyethoxy)ethylmethacrylate, ethoxytriethylene glycol methacrylate, methoxypolyethyleneglycol (molecular weight 200 to 1000) monomethacrylate, and polyethyleneglycol (molecular weight 200 to 1000) monomethacrylate; andethylenically unsaturated monomers having a hydroxyl group, such ashydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, andhydroxypentyl (meth)acrylate, hydroxyhexyl (meth)acrylate.

The monomer having a nonionic hydrophilic group is preferably anethylenically unsaturated monomer having an alkyl ether at the end,rather than an ethylenically unsaturated monomer having a hydroxyl groupat the end, from the viewpoints of the stability of the particles andthe content of the water-soluble component.

The hydrophilic constituent unit in the self-dispersing polymer ispreferably any of an embodiment containing only a hydrophilicconstituent unit having an anionic dissociative group, and an embodimentcontaining both a hydrophilic constituent unit having an anionicdissociative group and a hydrophilic constituent unit having a nonionichydrophilic group.

Furthermore, an embodiment containing two or more types of hydrophilicconstituent units having an anionic dissociative group, or an embodimenthaving two or more of a hydrophilic constituent unit having an anionicdissociative group and a hydrophilic constituent unit having a nonionichydrophilic group in combination, is also preferable.

The content of the hydrophilic constituent unit in the self-dispersingpolymer is preferably 25% by mass or less, more preferably from 1 to 25%by mass, further preferably from 2 to 23% by mass, and particularlypreferably from 4 to 20% by mass, from the viewpoints of viscosity andstability over time.

When the polymer has two or more types of hydrophilic constituent units,it is preferable that the total content of the hydrophilic constituentunit is within the range described above.

The content of the hydrophilic constituent unit having an anionicdissociative group in the self-dispersing polymer is preferably in therange such that the acid value falls in the suitable range describedbelow.

The content of the constituent unit having a nonionic hydrophilic groupis preferably from 0% by mass to 25% by mass, more preferably from 0% bymass to 20% by mass, and particularly preferably from 0% by mass to 15%by mass, from the viewpoints of ejection stability and stability overtime.

When the self-dispersing polymer has an anionic dissociative group, theacid value (mg KOH/g) is preferably 20 to 200, more preferably 22 to120, and particularly preferably 25 to 100, from the viewpoint ofself-dispersibility, content of the water-soluble component, andfixation properties when the polymer constitutes an ink composition. Theacid value is particularly preferably 30 to 80. When the acid value is20 or greater, the particles can be dispersed more stably, and when theacid value is 200 or less, the content of the water-soluble componentcan be reduced.

—Hydrophobic Constituent Unit—

The hydrophobic constituent unit in the self-dispersing polymer is notparticularly limited so long as it is derived from a hydrophobicgroup-containing monomer (hydrophobic monomer), and may be a constituentunit derived from a monomer containing one type of hydrophobic group, ormay be a constituent unit derived from a monomer containing two or moretypes of hydrophobic groups. The hydrophobic group is not particularlylimited, and may be any of a chain-like aliphatic group, a cyclicaliphatic group, and an aromatic group.

The hydrophobic monomer is preferably such that at least one is a cyclicaliphatic group-containing monomer, and more preferably a cyclicaliphatic group-containing (meth)acrylate (hereinafter, may be referredto as “alicyclic (meth)acrylate”), from the viewpoints of blockingresistance, abrasion resistance and dispersion stability.

The alicyclic (meth)acrylate is a compound including a structural sitederived from (meth)acrylic acid and a structural site derived fromalcohol, and having a structure containing at least one unsubstituted orsubstituted alicyclic hydrocarbon group (cyclic aliphatic group) in thestructural site derived from alcohol. The alicyclic hydrocarbon groupmay be the structural site derived from alcohol itself, or may be linkedto the structural site derived from alcohol via a linking group.

The “alicyclic (meth)acrylate” means a methacrylate or acrylate havingan alicyclic hydrocarbon group.

The alicyclic hydrocarbon group is not particularly limited so long asit contains a cyclic non-aromatic hydrocarbon group, and may be amonocyclic hydrocarbon group, a bicyclic hydrocarbon group, or apolycyclic hydrocarbon group having three or more rings.

Examples of the alicyclic hydrocarbon group include a cycloalkyl groupsuch as a cyclopentyl group or a cyclohexyl group, a cycloalkenyl group,a bicyclohexyl group, a norbornyl group, an isobornyl group, adicyclopentanyl group, a dicyclopentenyl group, an adamantyl group, adecahydronaphthalenyl group, a perhydrofluorenyl group, atricyclo[5.2.1.0²′⁶]decanyl group, a bicyclo[4.3.0]nonane, and the like.

The alicyclic hydrocarbon group may be further substituted with asubstituent. Examples of the substituent include an alkyl group, analkenyl group, an aryl group, an aralkyl group, an alkoxy group, ahydroxyl group, a primary amino group, a secondary amino group, atertiary amino group, an alkyl- or arylcarbonyl group, a cyano group,and the like.

The alicyclic hydrocarbon group may further form a condensed ring.

The alicyclic hydrocarbon group according to the invention preferablyhas 5 to 20 carbon atoms in the alicyclic hydrocarbon group moiety, fromthe viewpoint of viscosity or solubility.

The linking group that links the alicyclic hydrocarbon group and thestructural site derived from alcohol may be suitably an alkylene group,an alkenylene group, an alkynylene group, an arylalkylene group, analkylenoxy group, a mono- or oligoethylenoxy group, a mono- oroligopropylenoxy group, or the like, having 1 to 20 carbon atoms.

Specific examples of the alicyclic (meth)acrylate according to theinvention will be shown below, but the invention is not limited tothese.

Examples of monocyclic (meth)acrylate include cycloalkyl (meth)acrylateshaving a cycloalkyl group having 3 to 10 carbon atoms, such ascyclopropyl (meth)acrylate, cyclobutyl (meth)acrylate, cyclopentyl(meth)acrylate, cyclohexyl (meth)acrylate, cycloheptyl (meth)acrylate,cyclooctyl (meth)acrylate, cyclononyl (meth)acrylate, and cyclodecyl(meth)acrylate.

Examples of bicyclic (meth)acrylate include isobornyl (meth)acrylate,norbornyl (meth)acrylate, and the like.

Examples of tricyclic (meth)acrylate include adamantyl (meth)acrylate,dicyclopentanyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate,and the like.

These can be used individually, or as mixtures of two or more types.

Among these, at least one of the bicyclic (meth)acrylate and thepolycyclic (meth)acrylate having three or more rings is preferable, andat least one selected from isobornyl (meth)acrylate, adamantyl(meth)acrylate and dicyclopentanyl (meth)acrylate is more preferable,from the viewpoints of the dispersion stability of the self-dispersingpolymer particles, and fixability and blocking resistance of an imageformed.

According to the invention, the content of the constituent unit derivedfrom alicyclic (meth)acrylate contained in the self-dispersing polymerparticles is preferably 20% by mass to 90% by mass, more preferably 40%by mass to 90% by mass, and particularly preferably 50% by mass to 80%by mass, from the viewpoints of the stability of the self-dispersedstate, stabilization of particle shape in an aqueous medium due to thehydrophobic interaction between the alicyclic hydrocarbon groups, and adecrease in the amount of the water-soluble component due to anappropriate hydrophobization of particles.

When the content of the constituent unit derived from alicyclic(meth)acrylate is 20% by mass or more, fixation properties and blockingcan be improved. On the other hand, when the content of the constituentunit derived from alicyclic (meth)acrylate is 90% by mass or less, thestability of the polymer particles is improved.

The self dispersing polymer according to the invention can beconstituted to further include another constituent unit as thehydrophobic constituent unit if necessary, in addition to theconstituent unit derived from alicyclic (meth)acrylate. The monomerforming the other constituent unit is not particularly limited so longas it is a monomer capable of copolymerizing with the alicyclic(meth)acrylate and the hydrophilic group-containing monomer, and anyknown monomer can be used.

Specific examples of the monomer forming the other constituent unit(hereinafter, may be referred to as “other copolymerizable monomer”)include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl(meth)acrylate, isopropyl (meth)acrylate, n-propyl (meth)acrylate,n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate,hexyl (meth)acrylate, and ethylhexyl (meth)acrylate; aromaticring-containing (meth)acrylates such as benzyl (meth)acrylate andphenoxyethyl (meth)acrylate; stryrenes such as styrene, α-methylstyrene,and chlorostyrene; dialkylaminoalkyl (meth)acrylates such asdimethylaminoethyl (meth)acrylate; N-hydroxyalkyl (meth)acrylamides suchas N-hydroxymethyl (meth)acrylamide, N-hydroxyethyl (meth)acrylamide,and N-hydroxybutyl (meth)acrylamide; N-alkoxyalkyl (meth)acrylamidessuch as N-methoxymethyl (meth)acrylamide, N-ethoxymethyl(meth)acrylamide, N-(n-, iso-)butoxymethyl (meth)acrylamide,N-methoxyethyl (meth)acrylamide, N-ethoxyethyl (meth)acrylamide, andN-(n-, iso-)butoxyethyl (meth)acrylamide; and the like.

Among them, the other constituent unit is preferably at least one(meth)acrylate containing a chain-like alkyl group having 1 to 8 carbonatoms, from the viewpoint of the flexibility of the polymer skeleton orthe ease of control of the glass transition temperature (Tg) and fromthe viewpoint of the dispersion stability of the self-dispersingpolymer, and is more preferably a (meth)acrylate having a chain-likealkyl group having 1 to 4 carbon atoms, and particularly preferablymethyl (meth)acrylate or ethyl (meth)acrylate. Here, the chain-likealkyl group refers to an alkyl group having a linear or branched chain.

According to the invention, a (meth)acrylate containing an aromaticgroup can also be preferably used.

When an aromatic-containing (meth)acrylate is contained as the othercopolymerizable monomer, the content of the constituent unit derivedfrom the aromatic-containing (meth)acrylate is preferably 40% by mass orless, more preferably 30% by mass or less, and particularly preferably20% by mass or less, from the viewpoint of the dispersion stability ofthe self-dispersing polymer particles.

Furthermore, when a styrene-type monomer is used as the othercopolymerizable monomer, the content of the constituent unit derivedfrom the styrene-type monomer is preferably 20% by mass or less, morepreferably 10% by mass or less, and further preferably 5% by mass orless, from the viewpoint of stability when the self-dispersing polymeris made into particles, and it is particularly preferable that thepolymer does not include a constituent unit derived from a styrene-typemonomer.

Here, the styrene-type monomer refers to styrene, substituted styreneα-methylstyrene, chlorostyrene, or the like), or a styrene macromerhaving a polystyrene structural unit.

The other copolymerizable monomer in the self-dispersing polymer may beused individually, or in combination of two or more types.

When the self-dispersing polymer includes the other constituent unit,the content is preferably from 10% by mass to 80% by mass, morepreferably from 15% by mass to 75% by mass, and particularly preferablyfrom 20% by mass to 70% by mass. When two or more types of the monomerforming the other constituent unit are used in combination, the totalcontent is preferably in the range mentioned above.

The self-dispersing polymer according to the invention is preferably apolymer obtainable by polymerizing at least three types of an alicyclic(meth)acrylate, another copolymerizable monomer and a hydrophilicgroup-containing monomer, and more preferably a polymer obtainable bypolymerizing at least three types of an alicyclic (meth)acrylate, analkyl group-containing (meth)acrylate having a linear or branched chainhaving 1 to 8 carbon atoms, and a hydrophilic group-containing monomer,from the viewpoint of dispersion stability.

According to the invention, it is preferable that the content of the(meth)acrylate having a linear or branched alkyl group having 9 or morecarbon atoms, and the constituent unit having a substituent with highhydrophobicity, which is derived from an aromatic group-containingmacromonomer or the like, is substantially none, and it is morepreferable that the polymer does not include any of the constituentunits at all, from the viewpoint of dispersion stability.

The self-dispersing polymer according to the invention may be a randomcopolymer having the respective constituent units introducedirregularly, or may be a block copolymer having the respectiveconstituent units introduced regularly. If the first polymer is a blockcopolymer, the respective constituent units may be synthesized in acertain order of introduction, or the same constituent component may beused two or more times. However, it is preferable that the first polymeris a random copolymer, from the viewpoints of all-purpose usability andmanufacturability.

The range of molecular weight of the self-dispersing polymer accordingto the invention is preferably from 3000 to 200,000, more preferablyfrom 10,000 to 200,000, and further preferably from 30,000 to 150,000,in terms of weight average molecular weight. When the weight averagemolecular weight is 3,000 or more, the amount of the water-solublecomponent can be effectively suppressed. When the weight averagemolecular weight is 200,000 or less, the self-dispersion stability canbe enhanced.

Here, the weight average molecular weight can be measured by gelpermeation chromatography (GPC).

From the viewpoint of controlling the hydrophilicity and hydrophobicityof the polymer, the self-dispersing polymer according to the inventionis preferably a vinyl polymer which includes a structure derived from analicyclic (meth)acrylate at a copolymerization ratio of 20% by mass to90% by mass, and at least one of a structure derived from a dissociativegroup-containing monomer and a structure derived from a (meth)acrylatecontaining a chain-like alkyl group having 1 to 8 carbon atoms, and hasan acid value of from 20 to 120, a total content of the hydrophilicstructural units of 25% by mass or less, and a weight average molecularweight of from 3,000 to 200,000.

The first polymer is more preferably a vinyl polymer which includes astructure derived from a bicyclic (meth)acrylate or a polycyclic(meth)acrylate having three or more rings at a copolymerization ratio of20% by mass or more and less than 90% by mass, and a structure derivedfrom a (meth)acrylate containing a chain-like alkyl group having 1 to 4carbon atoms at a copolymerization ratio of 10% by mass or more and lessthan 80% by mass, and a structure derived from a carboxygroup-containing monomer at an acid value in the range of 25 to 100, andhas a total content of the hydrophilic structural unit of 25% by mass orless, and a weight average molecular weight of from 10,000 to 200,000.

Furthermore, the first polymer is particularly preferably a vinylpolymer which includes a structure derived from a bicyclic(meth)acrylate or a polycyclic (meth)acrylate having three or more ringsat a copolymerization ratio of 40% by mass or more and less than 80% bymass, and at least a structure derived from methyl (meth)acrylate orethyl (meth)acrylate at a copolymerization ratio of 20% by mass or moreand less than 60% by mass, and a structure derived from acrylic acid ormethacrylic acid at an acid value in the range of 30 to 80, and has atotal content of the hydrophilic structural unit of 25% by mass or less,and a weight average molecular weight of from 30,000 to 150,000.

The glass transition temperature of the self-dispersing polymer is notparticularly limited, but the glass transition temperature is preferably120° C. or higher, more preferably from 120° C. to 250° C., even morepreferably from 150° C. to 250° C. and particularly preferably from 160°C. to 200° C.

If the glass transition temperature of the self-dispersing polymer is120° C. or higher, the blocking resistance (particularly, under the hightemperature and high humidity conditions) may be improved. When theglass transition temperature is 250° C. or lower, the abrasionresistance of images is enhanced.

The glass transition temperature of the self-dispersing polymer can beappropriately controlled according to methods conventionally used. Forexample, the glass transition temperature of the self-dispersing polymercan be controlled to a desired range by appropriately selecting the typeof the polymerizable group of the monomer constituting theself-dispersing polymer, the type or the composition ratio of thesubstituent on the monomer, the molecular weight of the polymermolecule, or the like.

For the glass transition temperature (Tg) of the self-dispersing polymeraccording to the invention, a measured Tg that is obtainable by actualmeasurement is applied. Specifically, the measured Tg means a valuemeasured under conventional measurement conditions using a differentialscanning calorimeter (DSC) EXSTAR6220 (trade name) manufactured by SIINanotechnology, Inc.

However, if measurement is difficult due to degradation of the polymeror the like, a calculated Tg that is computed by the followingcalculation formula, is applied.

The calculated Tg is calculated by the following formula (1):1/Tg=Σ(X _(i) /Tg _(i))  (1)

Here, it is assumed that in the polymer serving as the object ofcalculation, n species of monomer components, with i being from 1 to n,are copolymerized. X, is the weight fraction of the i^(th) monomer(ΣX_(i)=1), and Tg_(i) is the glass transition temperature (absolutetemperature) of a homopolymer of the i^(th) monomer, provided that Σtakes the sum of i=1 to i=n. Furthermore, for the value of the glasstransition temperature of a homopolymer of each monomer (Tg,), thevalues given in Polymer Handbook (3^(rd) edition) (J. Brandrup, E. H.Immergut, (Wiley-Interscience, 1989)) are employed.

The I/O value of the self-dispersing polymer is not particularlylimited, but from the viewpoints of blocking resistance and thestability of the ink composition, the value is preferably from 0.20 to0.55, more preferably from 0.30 to 0.54, and even more preferably from0.40 to 0.50.

If the I/O value of the self-dispersing polymer is less than 0.20, thestability of the ink composition may be decreased. If the I/O value isgreater than 0.55, blocking resistance (particularly, under hightemperature and high humidity conditions) may be decreased.

The I/O value, which is also called as an inorganicity value/organicityvalue, is a value that deals with the polarity of various organiccompounds in an organic conceptual manner, and is one of functionalgroup contribution methods setting parameters to each functional group.

The I/O value is explained in detail in “Organic Conceptual Diagram” (byKoda Yoshio, published by Sankyo Publishing Co., Ltd. (1984) and thelike. The concept of the I/O value is to indicate the result of dividingthe properties of a compound into organic groups representing covalentbonding properties and inorganic groups representing ion bondingproperties, and rating every organic compound as a point on a Cartesiancoordinate system designated as an organic axis and an inorganic axis.

The inorganicity value is a value obtained by evaluating the magnitudeof the influence of various substituents or bonds carried by an organiccompound on the boiling point, and converting the magnitude into anumerical data based on the hydroxyl group. Specifically, when thedistance between the boiling point curve of a linear alcohol and theboiling point curve of a linear paraffin is taken in the vicinity of acompound of five carbon atoms, the result is about 100° C. Thus, theinfluence of one hydroxyl group is defined as 100 as a numerical value,and the value obtained by converting the influence of varioussubstituents or various bonds on the boiling point into a numericalvalue based on this value of 100, serves as the inorganicity value ofthe substituent carried by an organic compound. For example, theinorganicity value of a —COOH group is 150, and the inorganicity valueof a double bond is 2. Therefore, the inorganicity value of an organiccompound of a certain type means the sum of the inorganicity values ofvarious substituents, bonds and the like carried by the compound.

The organicity value is defined by taking a methylene group in themolecule as a unit, and defining the influence of a carbon atomrepresenting the methylene group on the boiling point as the reference.That is, when one carbon atom is added to a linear saturated hydrocarboncompound having around 5 to 10 carbon atoms, the average value of anincrease in the boiling point is 20° C. Thus, the organicity value ofone carbon atom is defined as 20 based on this value, and the value ofconverting the influence of various substituents or bonds on the boilingpoint based on this value of 20, serves as the organicity value. Forexample, the organicity value of a nitro group (—NO₂) is 70.

An I/O value approximating to zero represents that the organic compoundis non-polar (hydrophobic, high organicity), and a larger valuerepresents that the organic compound is polar (hydrophilic, highinorganicity).

According to the present invention, the I/O value of the self-dispersingpolymer means a value determined by the following method. The I/O value(=I value/O value) of each monomer constituting the self-dispersingpolymer is calculated based on the organicity (O value) and theinorganicity (I value) described in Koda Yoshio, “Organic ConceptualDiagram—Fundamentals and Applications” (1984), p. 13. For each of themonomers constituting the polymer, a product of the (I/O value) and (mol% in the polymer) was calculated, these products were summed, and thevalue obtained by rounding off at the third decimal place was defined asthe I/O value of the self-dispersing polymer.

As the method of calculating the inorganicity value of each monomer,generally a double bond is regarded as having an inorganicity of 2 uponaddition; however, since the double bond disappears afterpolymerization, a value that does not add the portion of double bond asthe inorganicity value of the monomers was used to calculate the I/Ovalue of the self-dispersing polymer used in the present invention.

According to the invention, a polymer having a desired I/O value can beconstructed by appropriately adjusting the structure and content of themonomers constituting the self-dispersing polymer.

Hereinafter, specific examples of the self-dispersing polymer will belisted as exemplary compounds, but the present invention is not limitedto these. The numbers in the parentheses represent the mass ratio of thecopolymerized components.

Methyl methacrylate/isobornyl methacrylate/methacrylic acid copolymer(20/72/8), glass transition temperature: 180° C., I/O value: 0.44

Methyl methacrylate/isobornyl methacrylate/methacrylic acid copolymer(40/52/8), glass transition temperature: 160° C., I/O value: 0.50

Methyl methacrylate/isobornyl methacrylate/dicyclopentanylmethacrylate/methacrylic acid copolymer (20/62/10/8), glass transitiontemperature: 170° C., I/O value: 0.44

Methyl methacrylate/dicyclopentanyl methacrylate/methacrylic acidcopolymer (20/72/8), glass transition temperature: 160° C., I/O value:0.47

For the calculation of the I/O value, the following values were used asthe I/O values of the monomers constituting the polymer.

Methyl methacrylate: 0.60, isobornyl methacrylate: 0.29, dicyclopentanylmethacrylate: 0.32, methacrylic acid 0.47

The method for producing a self-dispersing polymer according to theinvention is not particularly limited, and the polymer can be producedby copolymerizing a monomer mixture according to a known polymerizationmethod. Among such polymerization methods, it is more preferable toperform polymerization in an organic medium from the viewpoint ofdroplet ejection properties when formed into an ink composition, and asolution polymerization method is particularly preferable.

In regard to the method for producing the self-dispersing polymer of theinvention, the water-insoluble polymer as described above can beproduced by subjecting a mixture including a monomer mixture and ifnecessary, an organic solvent and a radical polymerization initiator, toa copolymerization reaction under an inert gas atmosphere.

The method for producing an aqueous dispersion of self-dispersingpolymer particles according to the invention is not particularlylimited, and an aqueous dispersion of self-dispersing polymer particlescan be obtained by a known method. The process of obtaining aself-dispersing polymer as an aqueous dispersion is preferably a phaseinversion emulsification method including the following process (1) andprocess (2).

Process (1): a process of obtaining a dispersion by stirring a mixturecontaining a water-insoluble polymer, an organic solvent, a neutralizingagent and an aqueous medium.

Process (2): a process of removing at least a portion of the organicsolvent from the dispersion.

The process (1) is preferably a treatment of first dissolving thewater-insoluble polymer in an organic solvent, slowly adding aneutralizing agent and an aqueous medium thereto, and mixing andstirring the mixture to obtain a dispersion. As such, when aneutralizing agent and an aqueous medium are added into a solution ofthe water-insoluble polymer dissolved in an organic solvent, aself-dispersing polymer particle having a particle size with higherstorage stability can be obtained without requiring a strong shearforce.

The method of stirring the mixture is not particularly limited, and anygenerally used mixing and stirring apparatus, or if necessary, adispersing machine such as an ultrasonic dispersing machine or a highpressure homogenizer can be used.

Preferable examples of the organic solvent include alcohol-basedsolvents, ketone-based solvents, and ether-based solvents.

Examples of the alcohol-based solvents include isopropyl alcohol,n-butanol, t-butanol, ethanol and the like. Examples of the ketone-basedsolvents include acetone, methyl ethyl ketone, diethyl ketone, methylisobutyl ketone, and the like. Examples of the ether-based solventsinclude dibutyl ether, dioxane, and the like. Among these organicsolvents, ketone-based solvents such as methyl ethyl ketone andalcohol-based solvents such as isopropyl alcohol are preferred.

It is also preferable to use isopropyl alcohol and methyl ethyl ketonein combination. When the solvents are used in combination,aggregation/precipitation or fusion between particles does not occur,and a self-dispersing polymer particle having a microparticle size withhigh dispersion stability can be obtained. This is thought to be becausethe polarity change upon phase inversion from an oil system to anaqueous system becomes mild.

The neutralizing agent is used to partially or entirely neutralize thedissociative groups so that the self-dispersing polymer can form astable emulsified or dispersed state in water. In the case where theself-dispersing polymer of the invention has an anionic dissociativegroup as the dissociative group, examples of the neutralizing agent tobe used include basic compounds such as organic amine compounds,ammonia, and alkali metal hydroxides. Examples of the organic aminecompounds include monomethylamine, dimethylamine, trimethylamine,monoethylamine, diethylamine, triethylamine, monopropylamine,dipropylamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethyl-ethanolamine, N,N-diethyl-ethanolamine,2-diethylamino-2-methyl-1-propanol, 2-amino-2-methyl-1-propanol,N-methyldiethanolamine, N-ethyldiethanolamine, monoisopropanolamine,diisopropanolamine, and triisopropanolamine, etc. Examples of the alkalimetal hydroxide include lithium hydroxide, sodium hydroxide andpotassium hydroxide. Among them, sodium hydroxide, potassium hydroxide,triethylamine, and triethanolamine are preferred from the viewpoint ofthe stabilization of dispersion of the self-dispersing polymer particlesof the invention into water.

These basic compounds are preferably used in an amount of from 5 mol %to 120 mol %, more preferably from 20 mol % to 100 mol %, and furtherpreferably from 30 mol % to 80 mol %, based on 100 mol % of thedissociative group. When the content is 15 mol % or more, an effect ofstabilizing the dispersion of particles in water is exhibited, and whenthe content is 80 mol % or less, an effect of reducing water-solublecomponents is obtained.

In the process (2), an aqueous dispersion of self-dispersing polymerparticles can be obtained by distilling off the organic solvent from thedispersion obtained in the process (1) by a conventional method such asdistillation under reduced pressure, to thereby bring about phaseinversion into an aqueous system. The organic solvent in the obtainedaqueous dispersion is substantially removed, and the amount of theorganic solvent is preferably 0.2% by mass or less, and more preferably0.1% by mass or less.

The average particle size of the self-dispersing polymer particlesaccording to the invention is preferably in the range of 1 nm to 100 nm,more preferably 3 nm to 80 nm, and further preferably 5 nm to 60 nm. Theaverage particle size is particularly preferably from 5 nm to 40 nm.With an average particle size of 1 nm or more, manufacturability isenhanced. Further, with an average particle size of 100 nm or less,storage stability is enhanced. Here, the average particle size means avolume average particle size.

The particle size distribution of the self-dispersing polymer particlesis not particularly limited, and the polymer particles may have a broadparticle size distribution or a mono-dispersed particle sizedistribution. Water-insoluble particles may also be used as mixtures oftwo or more types.

The average particle size and particle size distribution of theself-dispersing polymer particles can be measured using, for example, alight scattering method.

In the ink composition of the invention, the self-dispersing polymerparticles preferably exist in a form that does not substantially containa colorant.

The self-dispersing polymer particles of the invention have excellentself-dispersibility, and the stability of a dispersion of the polymeralone is very high. However, for example, since the function as aso-called dispersant for stably dispersing a pigment is not verysignificant, if the self-dispersing polymer according to the inventionis present in the ink composition in a form containing a pigment,consequently the stability of the ink composition as a whole may begreatly decreased.

The ink composition of the present invention may contain one type ofself-dispersing polymer particles alone, or may contain two or moretypes of such particles.

The content of the self-dispersing polymer particles in the inkcomposition of the invention is preferably from 1% by mass to 30% bymass, more preferably from 2% by mass to 20% by mass, and particularlypreferably from 2% by mass to 10% by mass, based on the ink compositionfor inkjet recording, from the viewpoint of the glossiness of images.

The content ratio of the coloring particles and the self-dispersingpolymer particles (coloring particles/self-dispersing polymer particles)in the ink composition of the invention is preferably from 1/0.5 to1/10, and more preferably from 1/1 to 1/4, from the viewpoint ofabrasion resistance of images.

(Other Additives)

The ink composition of the invention can further include other additivesif necessary, in addition to the components mentioned above.

Examples of the other additives according to the invention include knownadditives such as color fading inhibitor, emulsion stabilizer,permeation accelerator, ultraviolet absorber, preservative,mildew-proofing agent, pH adjusting agent, surface tension regulator,defoamer, viscosity adjusting agent, dispersant, dispersed stabilizer,anti-rust agent and chelating agent. These various additives may beadded directly after the preparation of the ink composition, or may beadded during the preparation of the ink composition. Specifically, theother additives and the like described in paragraphs [0153] to [0162] ofJP-A No. 2007-100071 are included.

The surface tension adjusting agent may be a nonionic surfactant, acationic surfactant, an anionic surfactant, a betaine surfactant or thelike.

The amount of addition of the surface tension adjusting agent ispreferably an amount of addition that adjusts the surface tension of theink composition to 20 mN/m to 60 mN/m, more preferably an amount ofaddition that adjusts the surface tension to 20 mN/m to 45 mN/m, andfurther preferably an amount of addition that adjusts the surfacetension to 25 mN/m to 40 mN/m, in order to spot the ink compositionsatisfactorily by the inkjet method. The surface tension of the inkcomposition can be measured, for example, using a plate method at 25° C.

Specific examples of the surfactant as a hydrocarbon type preferablyinclude anionic surfactants such as fatty acid salts, alkyl sulfuricacid ester salts, alkyl benzenesulfonates, alkyl naphthalenesulfonates,dialkyl sulfosuccinates, alkyl phosphoric acid ester salts,naphthalenesulfonic acid-formalin condensates and polyoxyethylene alkylsulfuric acid salts; and nonionic surfactants such as polyoxyethylenealkyl ether, polyoxyethylene alkyl allyl ether, polyoxyethylene fattyacid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fattyacid ester, polyoxyethylene alkyl amine, glycerin fatty acid ester andoxyethylene oxypropylene block copolymer. SURFYNOLS (trade name,products of Air Products & Chemicals) and OLFINE E1010 (trade name,surfactant, manufactured by Nisshin Chemical Industry Co., Ltd.) whichare an acetylene type polyoxyethylene oxide surfactant) are preferablyused. Furthermore, amine oxide type amphoteric surfactants such asN,N-dimethyl-N-alkyl amine oxide are preferred.

Additionally, materials described on pages (37) to (38) of JP-A No.59-157636 and Research Disclosure No. 308119 (1989) as surfactants canbe used.

When fluorocarbon (alkyl fluoride type) surfactants, siliconesurfactants or the like, such as those described in JP-A Nos.2003-322926, 2004-325707 and 2004-309806 are used, abrasion resistancecan be improved.

The surface tension regulator can be used as an antifoamer, and fluorinecompounds, silicone compounds, chelating agents represented by EDTA, andthe like can be used.

When the application of ink is carried out by the inkjet method, theviscosity of the ink composition of the invention is preferably in therange of 1 mPa·s to 30 mPa·s, more preferably in the range of 1 mPa·s to20 mPa·s, further preferably in the range of 2 mPa·s to 15 mPa·s, andparticularly preferably in the range of 2 mPa·s to 10 mPa·s, from theviewpoints of the droplet ejection stability and rate of aggregation.

The viscosity of the ink composition can be measured by, for example,Brookfield Viscometer at 20° C.

In the invention, the pH of the ink composition is preferably 7.5 to 10,and more preferably 8.0 to 9.5, from the viewpoints of the ink stabilityand rate of aggregation. The pH of the ink composition may be measuredusing a conventional pH measurement apparatus (for example, HM-30R;trade name, manufactured by DKK-TOA CORPORATION) at a temperature of 25°C. The pH of the ink composition is appropriately controlled by applyingan acidic compound or basic compound. A conventional acidic compound orbasic compound may be used as the acidic compound or basic compoundwithout any restriction.

In an image forming method according to the invention, an exemplaryembodiment of forming an image by using an ink set of the inventionwhich includes at least one of the ink compositions, and at least onetreatment liquid configured to form aggregates when contacted with theink composition, is preferable.

The ink set can be used in the form of an ink cartridge holding theseinks collectively or independently, and is preferable in view of theease of handling. The ink cartridge constituted to include the ink setis known in the related technical field, and can be prepared as an inkcartridge by appropriately using a known method.

(Treatment Liquid)

The treatment liquid in the invention is an aqueous composition whichforms an aggregate when contacted with the ink composition for inkjetrecording, and specifically, contains at least an aggregating componentwhich may aggregate the dispersed particles such as the coloringparticles (pigments) in the ink composition to form an aggregate and, ifnecessary, may contain other components. By using the treatment liquidtogether with the ink composition, inkjet recording may be speeded upand, even when high speed recording is performed, an image having highdensity and high resolution is obtained.

The treatment liquid contains at least one aggregating component whichforms an aggregate when contacted with the ink composition. By mixingthe treatment liquid into the ink composition ejected by an inkjetmethod, aggregation of a pigment or the like which has been stablydispersed in the ink composition is promoted.

Examples of the treatment liquid include a liquid composition which maygenerate an aggregate by changing the pH of the ink composition.Thereupon, the pH (25° C.) of the treatment liquid is preferably from 1to 6, more preferably from 1.2 to 5, and further preferably from 1.5 to4 from the viewpoints of the aggregation rate of the ink composition. Inthis case, the pH (25° C.) of the ink composition used in the ejectionstep is preferably 7.5 to 9.5 (more preferably 8.0 to 9.0).

In the invention, it is preferable that the pH (25° C.) of the inkcomposition is 7.5 or higher, and the pH (25° C.) of the treatmentliquid is 3 to 5, from the viewpoint of the image density, theresolution, and speeding-up of inkjet recording.

The aggregating component may be used alone, or two or more of them maybe used by mixing them.

The treatment liquid may be prepared by using at least one acidiccompound as the aggregating component. As the acidic compound, compoundshaving a phosphoric acid group, a phosphonic acid group, a phosphinicacid group, a sulfuric acid group, a sulfonic acid group, a sulfinicacid group, or a carboxy group, or salts thereof (e.g. polyvalent metalsalts) may be used. Among them, from the viewpoint of the aggregationrate of the ink composition, compounds having a phosphoric acid group ora carboxy group are more preferable, and compounds having a carboxygroup are further preferable.

The compound having a carboxy group is preferably selected frompolyacrylic acid, acetic acid, glycoric acid, malonic acid, malic acid,maleic acid, ascorbic acid, succinic acid, glutaric acid, fumaric acid,citric acid, tartaric acid, lactic acid, sulfonic acid, orthophosphoricacid, pyrrolidone carboxylic acid, pyrone carboxylic acid, pyrolecarboxylic acid, furan carboxylic acid, pyridine carboxylic acid,coumalic acid, thiophene carboxylic acid, nicotinic acid, or derivativesof such compound or salts thereof (for example, polyvalent metal salts,etc.) One of these compounds may be used alone or two or more of thesecompounds may be used together.

The treatment liquid in the invention may further include an aqueoussolvent (for example, water) in addition to the acidic compounddescribed above.

The content of the acidic compound in the treatment liquid is,preferably, from 5% by mass to 95% by mass and, more preferably, from10% by mass to 80% by mass based on the entire mass of the treatmentliquid from the viewpoint of aggregation effect.

Preferred examples of the treatment liquid that may improve the highspeed aggregation property include a treatment liquid including apolyvalent metal salt or a polyallyl amine. Examples of the polyvalentmetal salt and a polyallyl amine include salts of alkaline earth metalsbelonging to group 2 of the periodic table (for example, magnesium andcalcium), salts of a transition metal belonging to group 3 of theperiodic table (for example, lanthanum), salts of a cation of a metalbelonging to group 13 of the periodic table (for example, aluminum),salts of a lanthanide (for example, neodium), polyallylamine andpolyallylamine derivatives. As the metal salts, carboxylic acid salts(such as, salts of formic acid, salts of acetic acid, and salts ofbenzoic acid), nitric acid salts, chlorides, and thiocyanic acid saltsare preferred, and calcium salts or magnesium salt of a carboxylic acid(such as salts of formic acid, salts of acetic acid, and salts ofbenzoic acid), calcium salt of nitric acid or magnesium salt of nitricacid, calcium chloride, magnesium chloride, and calcium salt ofthiocyanic acid or magnesium salt of thiocyanic acid are more preferred.

The content of the metal salt in the treatment liquid is preferably from1% by mass to 10% by mass, more preferably, from 1.5% by mass to 7% bymass and, further preferably, from 2% by mass to 6% by mass.

The viscosity of the treatment liquid is, preferably, in a range from 1mPa·s to 30 mPa·s, more preferably, in a range from 1 mPa·s to 20 mPa·s,further preferably, in a range from 2 mPa·s to 15 mPa·s, and,particularly preferably, in a range from 2 mPa·s to 10 mPa·s from theviewpoint of the aggregation rate of the ink composition. The viscosityis measured by using VISCOMETER TV-22 (trade name, manufactured by TOKISANGYO CO., LTD.) under the condition at 20° C.

The surface tension of the treatment liquid is, preferably, from 20 mN/mto 60 mN/m, more preferably, from 20 mN/m to 45 mN/m and, furtherpreferably, from 25 mN/m to 40 mN/m from the viewpoint of theaggregation rate of the ink composition. The surface tension is measuredby using Automatic Surface Tensiometer CBVP-Z (trade name, manufacturedby Kyowa Interface Science Co. Ltd.) under the condition of being at 25°C.

EXAMPLES

Hereinafter, the present invention will be specifically described withrespect to Examples, but the present invention is not limited to theseExamples unless exceeds the subject matter of the invention. Unlessstated otherwise, the “parts” and “%” are based on mass.

The weight average molecular weight was measured by using a gelpermeation chromatography (GPC). HLC-8220 GPC (trade name, manufacturedby Tosoh Corp.) was used for the GPC, and TSKgeL SuperHZM-H, TSKgeLSuperHZ4000, and TSKgeL SuperHZ2000 (trade names, all manufactured byTosoh Corp.) were used as the columns and were connected in a series ofthree. The eluent liquid was THF (tetrahydrofuran). For the conditions,the sample concentration was 0.35% by mass, the flow rate was 0.35ml/min, the amount of sample injection was 10 μl, the measurementtemperature was 40° C., and an RI detector was used. A calibration curvewas produced from 8 samples of the 2 standard sample TSK standard,polystyrene”: “F-40”, “F-20”, “F-4”, “F-1”, “A-5000”, “A-2500”, “A-1000”and “n-propylbenzene” (trade names) manufactured by Tosoh Corp.

Example 1

<Production of Ink Composition>

—Synthesis of resin dispersant P-1—

88 g of methyl ethyl ketone was added to a 1000-mL three-necked flaskequipped with an agitator and a cooling tube, and was heated to 72° C.under a nitrogen atmosphere. To this, a solution of 0.85 g ofdimethyl-2,2′-azobisisobutyrate, 50 g of phenoxyethyl methacrylate, 11 gof methacrylic acid and 39 g of methyl methacrylate dissolved in 50 g ofmethyl ethyl ketone was added dropwise over 3 hours. After the additionwas completed, the mixture was reacted for one more hour, and then asolution of 0.42 g of dimethyl-2,2′-azobisisobutyrate dissolved in 2 gof methyl ethyl ketone was added. The temperature was raised to 78° C.,and the mixture was heated for 4 hours. Methyl ethyl ketone (MEK) wasadded to the obtained reaction solution to obtain 36.8% MEK solution ofa phenoxyethyl methacrylate/methyl methacrylate/methacrylic acidcopolymer (copolymerization ratio by mass percent=50/39/11) (resindispersant P-1).

The composition of the obtained resin dispersant P-1 was confirmed by¹H-NMR, and the weight average molecular weight (Mw) determined by GPCwas 49,400. The acid value of the copolymer was determined by the methoddescribed in JIS Standards (JIS K0070: 1992), and the value was 71.7mgKOH/g. The measured Tg of the copolymer (resin dispersant P-1) was 94°C.

—Synthesis of Resin Dispersant P-2—

240 g of methyl ethyl ketone, 30 g of a mixture ofN-(4-vinylbenzyl)-10H-acridin-9-one andN-(3-vinylbenzyl)-10H-acridin-9-one (mixture mass ratio 1 to 1), 20 g ofmethacrylic acid and 150 g of ethyl methacrylate were added to a 1000-mLthree-necked flask equipped with an agitator and a cooling tube, andheated to 75° C. under a nitrogen atmosphere. To this, a solution of2.44 g of dimethyl-2,2′-azobisisobutyrate dissolved in 16 g of methylethyl ketone, was added. The mixture was reacted with stirring whilemaintaining the temperature of 75° C. for two hours, followed by anaddition of a solution of 1.0 g of dimethyl-2,2′-azobisisobutyratedissolved in 2.0 g of methyl ethyl ketone and further reaction for twohours. To the mixture, a solution of 1.0 g ofdimethyl-2,2′-azobisisobutyrate dissolved in 2.0 g of methyl ethylketone was added. The temperature of the mixture was raised to 80° C.,and the mixture was heated for 4 hours. Methyl ethyl ketone (MEK) wasadded to the obtained reaction solution to obtain MEK solution of amixture of N-(4-vinylbenzyl)-10H-acridin-9-one andN-(3-vinylbenzyl)-10H-acridin-9-one (mixture mass ratio 1 to 1)/ethylmethacrylate/methacrylic acid copolymer (copolymerization ratio by masspercent=15/75/10) (resin dispersant P-2).

The measured Tg of the copolymer (resin dispersant P-2) was 124° C. Acontent of a non-volatile component in the obtain MEK solution of thecopolymer (resin dispersant P-2) was measured by weighing after drying apart of the obtain MEK solution by heating under reduced pressure. Thevalue was 36.8% by weight. The composition of the obtained resindispersant P-2 was confirmed by ¹H-NMR, and the weight average molecularweight (Mw) determined by GPC was 44,200. The acid value of thecopolymer was determined by the method described in JIS Standards (JISK0070: 1992), and the value was 65.2 mgKOH/g.

—Production of Self-dispersing Polymer Particles B-01—

560.0 g of methyl ethyl ketone was introduced into a two litterthree-necked flask equipped with an agitator, a thermometer, a refluxcooling tube and a nitrogen gas inlet tube, and the temperature wasincreased to 87° C. under a nitrogen atmosphere. While maintaining acondition of reflux in the reaction vessel (until finishing thereaction), a mixed solution formed from 220.4 g of methyl methacrylate(MMA), 301.6 g of isobornyl methacrylate (IBOMA), 58.0 g of methacrylicacid (MAA), 108 g of methyl ethyl ketone and 2.32 g of “V-601” (tradename, manufactured by Wako Pure Chemical Industries, Ltd.) was addeddropwise at a constant rate so that dropping would be completed in 2hours. After stirring the reaction mixture for one hour after theaddition was completed, a solution formed from 1.16 g of “V-601” and 6.4g of methyl ethyl ketone was added, and the mixture was stirred for 2hours (referred as a reaction step (1)). The reaction step (1) wasrepeated four times and then a solution formed from 1.16 g of “V-601”and 6.4 g of methyl ethyl ketone was further added, and the mixture wasstirred for 3 hours. The temperature was lowered to 65° C. afterperforming the polymerization reaction, and 163 g of isopropanol wasadded. The reaction mixture was rendered to cool in the atmosphere.

The weight average molecular weight (Mw) of the obtained copolymer was63,000, and the acid value was 65.1 (mg KOH/g).

Next, 317.3 g of the polymerized solution (solid content 41.0%) wasweighed, and 46.4 g of isopropanol, 1.65 g of a 20% aqueous solution ofmaleic anhydride (which is correspond to 0.3% by weight as maleic acidto the amount of the copolymer) and 40.77 g of a 2 mol/L aqueous NaOHsolution were added. The temperature in the reaction vessel wasincreased to 70° C. Subsequently, 380 g of distilled water was addeddropwise at a rate of 10 mL/min to achieve dispersion in water(dispersion step). Subsequently, 287.0 g of the solvent includingisopropanol, methyl ethyl ketone and water was distilled off under thereduced pressure, while holding for 1.5 hours at a temperature of 70° C.in the reactive vessel (solvent removing step). Then, 0.278 g of PROXELGXL(S) (trade name, manufactured by Arch Chemicals Japan Inc.) (whichcorresponds 440 ppm as benzoisothiazoline to a solid of the copolymer)was added. Then the resulting liquid was filtered with a filter having apore diameter of 1 μm to obtain a dispersion of a self-dispersingpolymer particle (B-01) at a solids concentration of 26.5%. The obtainedself-dispersing polymer particle was diluted with ion exchanged water toobtain aqueous dispersion of 25.0% concentration for measurement ofphysical properties. The obtained values for the physical propertieswere followings. a pH; 7.8, electric(al) conductivity; 461 mS/m,viscosity; 14.8 mPa·s, and volume average particle diameter; 2.8 nm.

<Measurement of Glass Transition Temperature Tg>

The glass transition temperature of the obtained polymer (particlesB-01) was measured by the following method, and was 160° C.

The polymer solution after polymerization in an amount of 0.5 g in termsof solid fraction was dried under reduced pressure at 50° C. for 4 hoursto obtain a polymer solid fraction. The obtained polymer solid fractionwas used to measure Tg by a differential scanning calorimeter (DSC)EXSTAR6220 (trade name) manufactured by SII Nanotechnology, Inc. Themeasurement conditions were such that 5 mg of a sample was sealed in analuminum pan, and the value of the peak top of DDSC from the measurementdata obtained at the time of second temperature increase in thefollowing temperature profile under a nitrogen atmosphere, wasdesignated as Tg.

from 30° C. to −50° C. (cooled at 50° C./min)

from −50° C. to 120° C. (heated at 20° C./min)

from 120° C. to −50° C. (cooled at 50° C./min)

from −50° C. to 120° C. (heated at 20° C./min)

<Measurement of Volume Average Particle Diameter (Mv)>

An aqueous dispersion of the resultant self-dispersible polymer particlewas arbitrarily diluted to the concentration (loading index of the rangeof 0.1 to 10) suitable for measurement, the volume average particlediameter of all aqueous dispersions was measured under same measurementconditions by a dynamic light scattering method, using an ULTRA FINEPARTICLE DIAMETER DISTRIBUTION MEASURER NANOTRACK UPA-EX150 (trade name,manufactured by Nikkiso Co., Ltd.). That is to say, it was measuredunder the following conditions: particle permeability of transmission,particle refractive index of 1.51, particle shape of nonsphere, densityof 1.2 g/cm³, water as the solvent, cell temperature of 18° C. to 25° C.

˜Production of Dispersion of Resin-coated Pigment Particle˜

(Production of Cyan Pigment Dispersion C)

100 g of Pigment Blue 15:3 (phthalocyanine blue A 220 wet cake (pigmentsolid content 33.5%), made from Dainichiseika Color & Chemicals Mfg.Co., Ltd.) as pigment solid content, 45 g of the phenoxy ethylmethacrylate/methyl methacrylate/methacrylic acid copolymer (resindispersant P-1) as the solid content, 140 g of methyl ethyl ketone, 50.6g of 1 mol/L aqueous sodium hydroxide solution (degree of neutralizationwith respect to methacrylic acid 88 mol %) as a pH adjuster, 331 g ofion exchanged water is dispersed with disperser in advance as a pigment,a further eight-pass process was performed by a disperser (trade name;MICROFLUIDIZER M-140K, manufactured by Microfluidic™ Corporation, 150MPa).

Subsequently, methyl ethyl ketone in the resultant dispersion wasremoved under reduced pressure at 56° C., a further 1 part of water wasremoved, a centrifugal treatment was performed at 8,000 rpm for 30minutes by a 50 mL centrifugal tube, using HIGH SPEED CENTRIFUGAL COOLER7550 (trade name, manufactured by Hisamitsu Pharmaceutical Co., Inc.),the supernatant solution, other than the precipitates, was collected.

Subsequently, the resultant dispersion (supernatant liquid) was heatedto 70° C. for 4 hours, and then 80 ppm of 2-methyl-4-isothiazolin-3-on,40 ppm of 5-chloro-2-methyl-isothiazolin-3-on, 10 ppm of2-bromo-2-nitropropan-1,3-diol, 30 ppm of 4,4-dimethyloxazolidine, 80ppm of 1,2-benzisothiazolin-3-on, and 30 ppm of2-n-octyl-4-isothiazolin-3-on as an antiseptic agent were added thereto,followed by filtration, and the filtrate was collected. The pigmentconcentration was calculated from the absorption spectrum, a pigmentconcentration of 15% resin-coated pigment particle dispersion (cyanpigment dispersion C) was obtained. The dispersion was pH 8.5 andviscosity of 2.9 mPa·s.

<Measurement of Particle Diameter of Resin-Coated Pigment Particle>

With respect to the resultant resin-coated pigment particle dispersion,the volume average particle diameter was measured by a dynamic lightscattering method, using PARTICLE DIAMETER DISTRIBUTION MEASURERNANOTRACK UPA-EX150 (trade name, manufactured by Nikkiso Co., Ltd.).Measurement was performed by adding 10 mL of the ion exchange water to10 pL of the resin-coated pigment particle dispersion to produce ameasurement sample liquid, followed by adjusting the temperature to 25°C. As a measurement result, volume average particle diameter of theresin-coated pigment particle was 88 nm.

(Production of Magenta Pigment Dispersion M)

100 g of Pigment Red 122 (CROMOPHTAL JET MAGENTA DMQ; trade name,manufactured by Chiba specialty corporation; Magenta pigment), 30 g ofthe resin dispersant P-2 as the solid content, 133 g of methyl ethylketone, 27.2 g of 1 mol/L aqueous NaOH solution (degree ofneutralization with respect to methacrylic acid 78 mol %), and 424 g ofion exchanged water were mixed, further dispersed by disperser mixing inadvance, and a 10-pass process was performed by a disperser(MICROFLUIDIZER M-140K; trade name, 150 MPa).

Subsequently, the same operation as performed for the cyan pigmentdispersion C was performed to obtain a pigment concentration of 15%resin-coated pigment particle dispersion (Magenta pigment dispersion M).Further, the volume average particle diameter, pH, and viscosity of theresultant dispersion using the same method as described above weremeasured to have a diameter of 76 nm, pH 8.6, and viscosity of 2.8mPa·s.

˜Production of Ink˜

(Production of Cyan Ink)

Each component was mixed so as to have the ink composition describedbelow, using cyan pigment dispersion C as obtained above and aself-dispersible polymer particle B-01. The resultant was charged by adisposable syringe made of a plastic. The resultant was filtrated withPVDF 5 μm filter (trade name; MILLEX-SV, diameter of 25 nm, manufacturedby Millipore corporation) to produce cyan ink (ink composition forinkjet) C-01.

<Composition of Cyan Ink> Cyan pigment (Pigment Blue 15:3) 2.5%   Theresin dispersant P-1 (solid content) 1.125%    The self-dispersiblepolymer particle B-01 8.5%   (solid content) Colloidal silica (solidcontent) 0.05%   (trade name; SNOWTEX XS, volume average particlediameter: 4 to 6 nm, manufactured by Nissan Chemical Industries, Ltd.)SUNNIX GP 250 8% (trade name, manufactured by Sanyo Chemical Industries,Ltd., hydrophilic organic solvent, I/O value 1.30) Tripropylene glycolmonomethyl ether (TPGmME) 8% (trade name; MFTG, manufactured by NipponNyukazai Co., Ltd., hydrophilic organic solvent, I/O value 0.80) Urea(manufactured by Nissan Chemical 5% Industries, Ltd., solid wettingagent) NEWPOLE PE-108 (trade name, manufactured 0.15%   by SanyoChemical Industries, Ltd., thickening agent) OLFINE E1010 (trade name,manufactured by 1% Nissin Chemical Industry Co., Ltd., surfactant) Ionexchanged water remainder (up to the total amount of 100%)

(Production of Magenta Ink)

Magenta ink (ink composition for inkjet) M-01 was produced in a mannersubstantially same as that in production of the cyan ink C-01 exceptthat each component was mixed so as to have the ink compositiondescribed below, using the magenta pigment dispersion M as obtainedabove and a resin dispersant P-2.

<Composition of Magenta Ink> Magenta pigment (Pigment Red 122) 5.0%  The resin dispersant P-2 (solid content) 1.5%   The self-dispersiblepolymer particle B-01 7.25%   (solid content) SUNNIX GP 250 10%  (tradename, manufactured by Sanyo Chemical Industries, Ltd., hydrophilicorganic solvent, I/O value 1.30) Tripropylene glycol monomethyl ether 6%(TPGmME) (trade name; MFTG, manufactured by Nippon Nyukazai Co., Ltd.,hydrophilic organic solvent, I/O value 0.80) Urea (manufactured byNissan Chemical 5% Industries, Ltd., solid wetting agent) NEWPOLE PE-108(trade name, manufactured 0.05%   by Sanyo Chemical Industries, Ltd.,thickening agent) OLFINE E1010 (trade name, manufactured by 1% NissinChemical Industry Co., Ltd., surfactant) Ion exchanged water remainder(up to the total amount of 100%)

Production of Treatment Liquid

Each component was mixed so as to have the composition described belowto produce the treatment liquid T-1. The viscosity and surface tensionof the obtained treatment liquid was measured by the same method asdescribed above to have a viscosity of 2.3 mPa·s, surface tension of42.5 mN/m, and pH 1.0.

<Composition> malonic acid (manufactured by Tateyama Kasei Co., Ltd.)11.3% DL-malic acid (manufactured by Fuso Chemical Co., Ltd.) 14.5%Diethylene glycol monobutyl ether   4% (trade name; BDG, manufactured byNippon Nyukazai Co., Ltd.) Tripropylene glycol monomethyl ether   4%(trade name; MFTG, manufactured by Nippon Nyukazai Co., Ltd.) Ionexchanged water 66.2%<Image Forming and Evaluation>˜Image Forming˜

An Inkjet head 1 constituted as shown in FIGS. 6 to 8 and with thesilicon nozzle plate was prepared, and the magenta ink as obtained abovewas charged to the connected storage tank thereto. The silicon nozzleplate is formed of single crystal silicon, and a silicon oxide film(SiO₂ film) is formed on the surface thereof at a side toward the inkejection direction of the nozzle by a CVD method by introducing SiCl₄and water vapor to a chemical vapor deposition (CVD) reactor. Thethickness of SiO₂ film is 50 nm. Further, after performing an oxygenplasma process, chemical vapor deposition (CVD) was performed usingC₈F₁₇C₂H₄SiCl₃, and the liquid repellent film was formed on SiO₂ film.The liquid repellent film was formed by introducing C₈F₁₇C₂H₄SiCl₃ andwater vapor at the low pressure into CVD reactor. The thickness of theliquid repellent film is 10 nm. Further, plural nozzles as shown inFIGS. 2 to 4 are arranged two-dimensionally in a matrix form in thesilicon nozzle plate, and ink droplets can be ejected with highprecision as shown in FIG. 5. As a recording medium, TOKUBISHI ART BOTHFACES N (trade name, manufactured by Mitsubishi Paper Mills, Ltd.) wasprepared.

The recording medium was fixed on a transferable stage in thepredetermined straight line direction at 500 mm/second, stagetemperature was held at 30° C., the treatment liquid as obtained abovewas coated at a thickness of about 1.2 μm with a bar coater, followed bydrying at 50° C. for 2 seconds immediately after coating. The preparedinkjet head was fixed and disposed such that the line direction (w, 310,320, 330, 340 etc. in FIG. 5) where nozzle was aligned to an inclinationof 75.7° (90°−α in FIGS. 2 and 5) with respect to the direction(principal scanning direction) orthogonal to the movement direction(sub-scanning direction) of the stage. While the recording medium wasmoved at the constant speed in the sub-scanning direction, ink wasejected linearly under conditions of an ink droplet volume of 6.0 pL,ejection frequency of 25.7 kHz, resolution of 1200 dpi×1200 dpi, and animage was recorded which contained a 50% solid image with an area of 2square cm, a 4 to 8 pt image of the character a

(TODOROKI)“, of and a 4 pt image of the character a

(TODOROKI)” of as a white letter on a solid image.

Immediately after recording, the image was passed between a pair offixing rollers, which were dried at 60° C. for 3 seconds and at the 60°C., and fixing process was performed at a NIP pressure of 0.25 MPa and aNIP width of 4 mm to obtain an evaluation sample.

˜Image Evaluation-1˜

—1. Resolution of Image—

Among image of the resultant evaluation sample, the resolution wasevaluated according to evaluation criteria described below by visualobservation with respect to a 4 to 8 pt image of the character a

(TODOROKI)” and a 4 pt image of the character

(TODOROKI)” of as a white letter on a solid image. The evaluationresults are shown in Table 2 below.

<Evaluation Criteria>

-   AA: Resolution is good for a 4 pt character, and the resolution is    at a level having no problems in practical application.-   A: The decrease of resolution was recognized at a part of the 4 pt    characters, but the resolution is at a level having no problems in    practical application.-   B: The decrease of resolution is recognized even in characters    larger than 4 pt and the resolution was at a level having low    practicality.-   C: The character is lost and the decrease of resolution was    prominent, and the resolution was at a level having extremely low    practicality.

—2. Image Density—

The density of the image section of the obtained evaluation sample wasmeasured using Reflection Densitometer (trade name; XRITE 938,manufactured by X-rite corporation) and was evaluated by the evaluationcriteria described below. The evaluation results are shown in Table 2below.

<Evaluation Criteria>

-   AA: Sufficient density is obtained, and the density is of an    extremely good level.-   A: Density is obtained, and the density is of a good level.-   B: Practical application presents no problem at this level.-   C: Density is reduced, and the density is at a level having low    practicality.-   D: Density is highly reduced and the density is at a level having    very low practicality.

—3. Head Reliability—

The inkjet head was continuously ejected at 25.7 kHz for 6000 hundredmillion times, and then image is recorded, and line image of 75×2400 dpiwas drawn at an ejection frequency of 25.7 kHz using 96 nozzles. Withrespect to the line image, the center value of the line was measuredusing a DOT ANALYZER DA-6000, trade name, manufactured by Oji ScientificInstruments Co., Ltd., and a standard deviation σ of misalignment ofeach line was calculated. The evaluation results are shown in Table 2below.

<Evaluation Criteria>

-   AA: σ<2 μm-   A: 2 μm≦σ<3 μm-   B: 3 μm≦σ<5 μm-   C: 5 μm≦σ<7 μm-   D: 7 μm≦σ

—4. Nozzle Deformation—

The inkjet head was continuously ejected at 25.7 kHz for 6000 hundredmillion times, and then image is recorded, shape of 2048 nozzle holes, aliquid repellent film around the nozzle hole, and defect of an oxidefilm were observed by using an optical microscope. Shape of nozzle holeor change around the nozzle hole was evaluated according to evaluationcriteria described below. The evaluation results are shown in Table 2below.

<Evaluation Criteria>

-   AA: All nozzle holes are normal.-   A: Shape change of nozzle hole or change around nozzle hole is in    less than 5 locations-   B: Shape change of nozzle hole or change around the nozzle hole is    in 5 locations or more and less than 10 locations.-   C: Shape change of nozzle hole or change around the nozzle hole is    in 10 locations or more and less than 20 locations.-   D: Shape change of nozzle hole or change around the nozzle hole is    in 20 locations or more.

Example 2

An image was recorded and evaluated in a manner substantially same asthat in Example 1 except that the inkjet head 1 in Example 1 wasreplaced with the inkjet head 2 of configuration which have a siliconnozzle plate but did not have a rear flow path, that is to say, aconfiguration where the common liquid chamber was disposed on the sameside as the pressure chamber. Plural nozzles were provided in thesilicon nozzle plate as shown in FIGS. 2 to 4, and ink droplets withhigh precision can be ejected as shown in FIG. 5. Further, the siliconnozzle plate was formed of single crystal silicon, and silicon oxidefilm (SiO₂ film) was formed on the surface of the single crystal siliconat a side toward the ink ejection direction of the nozzle by chemicalvapor deposition (CVD). Further, Chemical vapor deposition (CVD) wasperformed using C₈F₁₇C₂H₄SiCl₃ after performing an oxygen plasma processand the liquid repellent film was formed on SiO₂ film. The evaluationresults are shown in Table 2 below.

Example 3

An image was recorded and evaluated in a manner substantially same asthat in Example 1 except a self-dispersible polymer particle dispersionB-01 used in ink production in Example 1 was replaced with a polymerparticle dispersion C obtained by an emulsion polymerization process asdescribed below. The evaluation results are shown in Table 2 below.

Production of Polymer Particle Dispersion C

To a 1 L three-necked flask equipped with stirrer and reflux condenserwere placed 8.1 g of PIONIN A-43s (trade name, manufactured by TakemotoOil & Fat Co., Ltd., emulsifier) and 236.0 g of distillated water,followed by heat and stirring at 70° C. under nitrogen gas flow. 6.2 gof styrene, 3.5 g of n-butyl acrylate, 0.3 g of acrylic acid, 1.0 g ofammonium persulfate, and 40 g of distillated water were added thereto,and after stirring for 30 minutes, dropwise addition was performed at asteady speed such that this dropwise addition of a monomer solutionconsisting of 117.8 g of styrene, 66.5 g of n-butyl acrylate and 5.7 gof acrylic acid completes in 2 hours. After completion of the dropwiseaddition, a water solution consisting of 0.5 g of ammonium persulfateand 20 g of distillated water was added thereto, followed by stirring at70° C. for 4 hours, and then the temperature was raised to 85° C. andwas stirred for 2 hours. A reaction solution was cooled down, andneutralization degree was neutralized to be 0.5 using 2 mol/L aqueousNaOH solution. Through successive filtration, a polymer particle BH-1dispersion was obtained represented by the structure below. The physicalproperties of the obtained polymer particle have weight-averagemolecular weight of 232,000, acid value of 23 mgKOH/g, and volumeaverage particle diameter of 70 nm.

Example 4

An image was recorded and evaluated in a manner substantially same asthat in Example 3 except that the inkjet head 1 in Example 3 wasreplaced with the inkjet head 2 of configuration which had a siliconnozzle plate but did not have a rear flow path, that is to say, aconfiguration where the common liquid chamber was disposed on the sameside as the pressure chamber. Plural nozzles were provided in thesilicon nozzle plate as shown in FIGS. 2 to 4, and ink droplets could beejected with high precision as shown in FIG. 5. Further, the siliconnozzle plate was formed of single crystal silicon, and silicon oxidefilm (SiO₂ film) was formed on the surface of the single crystal siliconat a side toward the ink ejection direction of the nozzle by chemicalvapor deposition (CVD). Further, chemical vapor deposition (CVD) wasperformed using C₈F₁₇C₂H₄SiCl₃ after performing an oxygen plasma processand liquid repellent film was formed on SiO₂ film. The evaluationresults are shown in Table 2 below.

Comparative Example 1

An image was recorded and evaluated in a manner substantially same asthat in Example 1 except that colloidal silica used in the inkproduction in Example 1 was not contained. The evaluation results areshown in Table 2 below.

Comparative Example 2

An image was recorded and evaluated in a manner substantially same asthat in Comparative Example 1 except that the inkjet head 1 inComparative Example 1 was replaced with the inkjet head 2 ofconfiguration which had a silicon nozzle plate but did not have a rearflow path, that is to say, a configuration where the common liquidchamber was disposed on the same side as the pressure chamber. Pluralnozzles were provided in the silicon nozzle plate as shown in FIGS. 2 to4, and ink droplets could be ejected with high precision as shown inFIG. 5. Further, the silicon nozzle plate was formed of single crystalsilicon, and silicon oxide film (SiO₂ film) was formed on the surface ofthe single crystal silicon at a side toward the ink ejection directionof the nozzle by chemical vapor deposition (CVD). Further, Chemicalvapor deposition (CVD) was performed using C₈F₁₇C₂H₄SiCl₃ afterperforming an oxygen plasma process and liquid repellent film was formedon SiO₂ film. The evaluation results are shown in Table 2 below.

Comparative Example 3

An image was recorded and evaluated in a manner substantially same asthat in Example 1 except that the inkjet head 1 used in Example 1 wasreplaced with the comparative head 3 which had a configuration which didnot have a rear flow path, that is to say, a configuration where thecommon liquid chamber was disposed on the same side as the pressurechamber, in which the nozzle plate was formed of stainless alloy(SUS316L), and the nozzle was not arranged two-dimensionally in a matrixform. In this regard, recording was performed by recording ejectionconditions of ink droplet volume of 2.4 pL, ejection frequency of 25.7kHz, and resolution of 300 dpi×300 dpi. The evaluation results are shownin Table 2 below.

Comparative Example 4

An image was recorded and evaluated in a manner substantially same asthat in Example 1 except that the inkjet head 1 in Example 1 wasreplaced with the comparative head 3 which had a configuration which didnot have a rear flow path, that is to say, a configuration where thecommon liquid chamber was disposed on the same side as the pressurechamber, in which the nozzle plate was formed of stainless alloy(SUS316L), and the nozzle was not arranged two-dimensionally in a matrixform, as well as colloidal silica which was used in ink production wasnot contained. In this regard, recording was performed by recordingejection conditions of ink droplet volume of 2.4 pL, ejection frequencyof 25.7 kHz, and resolution of 300 dpi×300 dpi. The evaluation resultsare shown in Table 2 below.

TABLE 1 Comparative Head 1 Head 2 Head 3 Substrate of nozzle plateSilicon Silicon SUS Protective film of nozzle plate Silicon oxideSilicon oxide none Piezoelectric body disposed disposed disposedTwo-dimensional matrix form arranged arranged none (FIGS. 2 to 5) (FIGS.2 to 5) Rear face flow path design adopted none none Resolution 1200 dpi1200 dpi 300 dpi (single-pass) (single-pass) (single-pass)

TABLE 2 Colloidal Image Image Head Nozzle Inkjet Head Silica ResinParticles Resolution Density Reliability Deformation Example 1 Head 1used Self-dispersing AA AA AA AA Polymer Example 2 Head 2 usedSelf-dispersing A A AA AA Polymer Example 3 Head 1 used Emulsion A AA AAAA polymerized Example 4 Head 2 used Emulsion B B AA AA polymerizedComparative Head 1 none Self-dispersing AA AA D C Example 1 PolymerComparative Head 2 none Self-dispersing A A C C Example 2 PolymerComparative Comparative used Self-dispersing C C A A Example 3 Head 3Polymer Comparative Comparative none Self-dispersing C C A A Example 4Head 3 Polymer

As shown in Table 2, in Examples, deterioration of the head plate wassuppressed, nozzle deformation was at least excellent in headreliability, and higher precise images were stably formed. With respectto this, deterioration of head plate was not suppressed, but headreliability was reduced in Comparative Examples 1 to 2 which do notcontain colloidal silica. Further, in a conventional head plate whichdoes not use silicon as Comparative Examples 3 to 4, the resolution ofthe nozzle alignment itself cannot be improved and the resolution anddensity of the recorded image were insufficient.

Examples 5 to 8

An image was recorded and evaluated in the substantially same as that inExamples 1 to 4 except that magenta ink used Examples 1 to 4 was changedto cyan ink. As a result, in all Examples, deterioration of the headplate was suppressed in a manner substantially same as that in Examples1 to 4, nozzle deformation was at least excellent in head reliability,and higher precise images were stably formed.

Examples 9 to 12

An imaged was recorded and evaluated in a manner substantially same asthat in Examples 1 to 4 except that 0.05% colloidal silica (SNOWTEX)used in Examples 1 to 4 was replaced with 0.05% sodium silicate.

As a result, deterioration of head plate was suppressed in a mannersubstantially same as that in Examples 1 to 4, nozzle deformation was atleast excellent in head reliability, and higher precise images werestably formed.

The invention provides an image forming method and an ink compositionwhere deterioration of the head plate can be suppressed and higherprecise images were stably formed.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. The embodiments were chosenand described in order to best explain the principles of the inventionand its practical applications, thereby enabling others skilled in theart to understand the invention for various embodiments and with thevarious modifications as are suited to the particular use contemplated.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if such individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference. It will be obvious to those having skill inthe art that many changes may be made in the above-described details ofthe preferred embodiments of the present invention. It is intended thatthe scope of the invention be defined by the following claims and theirequivalents.

What is claimed is:
 1. An image forming method comprising: dischargingan ink composition from an inkjet head to form an image, the inkcomposition comprising an inorganic silicate compound, and the inkjethead being equipped with a nozzle plate at least a part of which isformed of silicon, wherein at least a part of the nozzle plate is coatedwith a film which comprises at least one selected from the groupconsisting of a metal oxide, a metal nitride and a metal other thansilicon, and a content of the inorganic silicate compound in the inkcomposition is 0.0005% by mass to 0.5% by mass with respect to a totalmass of the ink composition.
 2. The image forming method according toclaim 1, wherein at least a part of the nozzle plate is coated with afilm which comprises SiO₂ or tantalum oxide.
 3. The image forming methodaccording to claim 1, wherein the nozzle plate has plural ejection portswhich eject the ink composition, the inkjet head further comprisesplural pressure chambers respectively communicating with the pluralejection ports of the nozzle plate, plural ink supply flow pathsrespectively supplying the ink composition to the plural pressurechambers, a common liquid chamber supplying the ink composition to theplural ink supply flow paths, and plural pressure generation unitsrespectively deforming the plural pressure chambers, and an amount ofchange in volume within each pressure chamber is controlled by drivingthe respective pressure generation unit to eject the ink composition. 4.The image forming method according to claim 3, wherein the pressuregeneration units are piezo elements.
 5. The image forming methodaccording to claim 3, wherein the plural ejection ports are arrangedtwo-dimensionally in a matrix form.
 6. The image forming methodaccording to claim 5, wherein the inkjet head forms an image at adrawing resolution of 1200 dpi or higher with a single pulse ejectionfrom the nozzle plate.
 7. The image forming method according to claim 3,further including electrical wiring which is arranged so as to penetratethe common liquid chamber and supplies driving signals to the pressuregeneration units.
 8. The image forming method according to claim 7,wherein the pressure generation units are disposed on the opposite sideof the pressure chamber from a side thereof where the nozzle plate isarranged and the common liquid chamber is disposed on the opposite sideof the pressure generation units from a side thereof where the pressurechamber are arranged.
 9. The image forming method according to claim 8,wherein at least a part of the nozzle plate is coated with a film whichcomprises SiO₂ or tantalum oxide.
 10. The image forming method accordingto claim 1, wherein the ink composition further comprises a pigment, awater-soluble organic solvent, and resin particles.
 11. The imageforming method according to claim 10, wherein the resin particles areself-dispersing polymer particles.
 12. The image forming methodaccording to claim 11, wherein at least a part of the nozzle plate iscoated with a film which comprises SiO₂ or tantalum oxide.
 13. The imageforming method according to claim 1, wherein the inorganic silicatecompound is colloidal silica.
 14. The image forming method according toclaim 1, further comprising applying a treatment liquid on a recordingmedium, the treatment liquid being capable of forming an aggregate uponcontact with the ink composition.
 15. An ink composition comprising aninorganic silicate compound and being used for the image forming methodaccording to claim 1.